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Why do some animals have horizontal tail fins and others have vertical tail fins?	Only aquatic mammals have horizontal flukes/tails. Fish, reptiles and amphibians are vertically oriented. If you watch a shark (a fish), and alligator (a reptile) or a salamander (amphibian) swim, it moves its body side to side while a dolphin, whale, seal, or manatee (all mammals) moves up and down. Thus a fish needs its tail to be vertical while a mammal's most be horizontal. As to why that is.... evolution. But largely because mammals have their legs under them and so their spine curls in an up/down manner which is necessary for their locomotion; and whales and dolphins evolved from land-dwelling mammals. That's the very basic answer.
If two cars are traveling at 100 mph towards each other would the impact be the same as one car hitting a wall at 200 mph?	You mean in total? Because in one case you have one car (and a wall that would escape relatively unscathed) and the other case you have two cars. Or do you mean to the passengers in the cars? In this case the 200 mph case would be worse. The total speed makes no difference. What matters is the momentum change to a single passenger. In both cases they go from top speed to zero. In the first case the top speed is half the second case's top speed, so there would be a greater momentum change felt by the passenger in the second case. If you had asked which was worse - two cars going 100 mph hitting head-on or two cars each hitting a wall at 100 mph - the answer would be, in a first-year physics way, they would be equivalent. The momentum change felt by all passengers would be the same.
Why is it that if two cars run into each other while both going 60mph the collision isn't equivalent to a 120mph crash?	That 60 mph crash is into a solid, unmovable wall, and all the energy is absorbed by 1 car. In the two cars colliding head on at 60mph each, each car delivers as much energy as the other, but also absorbs as much, so while the total energy in the collision might be double, it's also absorbed by double the number of cars, and each car absorbs the same as just 1 hitting a solid object. When the cars have different masses, it changes, and something like a big rig at 60mph smoking a smart car at 60mph, head on, becomes much more like a 120mph accident, for the smart car. On the other hand, the big rig won't see it as nearly a violent accident as it would had it struck a solid wall instead. This has great implications in crash-test ratings. The Smart has great forward crash test rankings, thanks to it's high-tech and ultra-rigid body. But against something like an F-350...mass wins.
If the universe started from a singularity and is expanding outwards, how is it possible for two galaxies to collide?	Astrophysics dude here. You start from the false visualisation of the universe starting at a point, and expanding outwards like some kind of an explosion. This common faulty visualisation is likely due to the phrase "big bang", and is what gives rise to the confusing and incorrect question "what is it expanding into?" You need to get this out of your head; space itself is expanding. So every point in space is moving away from every other point. There is no "centre" of the expansion. Everywhere and nowhere is the centre at the same time. Locally, gravity can still attract two things together- hence you falling out of bed rather than being carried away from your bed by the expansion of the universe. So within a region of space, galaxies can and will move toward each other, as will stars, planets, and baseballs. In general and on a large scale, though, the universe is expanding and accelerating apart. One day in the (very) distant future, there will be no collisions.
Why do photons obey the law of equal angles, when reflected off of a mirror?	More generally, it's a consequence of the principle of least action. This can be applied to light to give us Fermat's principle, that light always takes the shortest path between two points. From there it's a matter of plane geometry to show that upon reflection, this shortest distance occurs precisely when the angles are equal. Slight caveat, Fermat's principle isn't exactly true in all situations and sometimes we have to replace "shortest path" with "extremal path" (that is, either minimum or maximum time) and "ordinary length" with "optical path length" For most cases below an undergeaduate level the above description should suffice though. If you prefer the wavefront interpretation, you can picture it in terms of constructive and destructive interference but I find it more difficult to picture mentally under this approach
Where did the heavy elements that formed the planets in our solar system originally come from?	Nuclear fusion in stars produces all the elements up to iron - it requires an input of energy for elements heavier than iron to form. They are only formed during the massive energy output of a supernova (not a nova - a nova is just an outburst of surface fusion, it doesn't destroy the star). You asked how this enriched gas gets to our solar system: gas flows out from stars, novae, and in particular supernovae, so that the surrounding region now has more heavy elements in it (we call this a "higher metallicity"). This "metal-rich" gas mixes in with the rest of the gas in the galaxy (the "interstellar medium") so the net effect is that the gas of the Milky Way is just a little higher metallicity. New stars form from this enriched gas, and will have more heavy elements than the previous stars.
Would my garden hose provide enough kerosene to a jet engine?	From figures I found online, that engine uses 3750 gallons per hour at takeoff thrust. Thrust during normal operation will be lower, as will fuel usage, but during takeoff that's around 4.5 litres per second. A garden hose spits about about 0.15 litres per second. So you'd need 30 hoses. Per engine.
Are galaxies moving away from each other due to the expansion of the universe? If so then why do galaxy collisions occur?	The expansion only really makes sense on large cosmic scales. Smaller structures, like our local group of galaxies, are small and dense enough that they've broken away from the expansion, and the velocities of galaxies within them are dominated by random motion rather than by the expansion. So while a distant galaxy like the [Sombrero Galaxy](_URL_0_) is expanding from us in accordance with Hubble's law, a neighbor like the [Andromeda Galaxy](_URL_1_) is actually on a collision course with the Milky Way!
If the universe is expanding, does that mean matter is being created? In other words, how is the universe expanding?	The space between the majority of objects is increasing. Matter itself is not being created.
Why don't people have different natural hair colors like blue or green?	Mammals main source of coloring is melanin, which produces browns, blacks and shade of tan. Other animals such as birds have multiple types of pigment along with melanin including, carotenoids which produce red, orange or yellow feathers. Also Porphyrins, an amino acid that can produce red, brown, pink and green colors. The source of these pigments comes from many of the foods that birds eat. So the way a birds body digests and converts these substances is another thing that our bodies did not evolve to do. The forth material that produces colors like blue and green is keratin. Our hair is made up of this same material although the way keratin is used in other animals is a bit different. The structure of the keratin reflects certain light waves which makes it appear blue. The color of the keratin isn't blue itself. A birds feathers on a microscopic level can have a structure that reflects blue light. Butterfly wings also work in this way. Our hair doesn't have this complex structure to do the same.
How can cell phones possibly cause cancer if they emit microwaves, which are a form of non-ionizing radiation?	That's an oft-repeated but a bit _too_ simplistic argument. Yes, since microwaves aren't ionizing radiation, they can't cause cancer in the same way that ionizing radiation does, and therefore any comparisons to "radiation" (in the nuclear/gamma kind of sense) are entirely wrong and misleading. But non-ionizing radiation can also cause cancer - UV light for instance. But microwave radiation isn't comparable to that either, it has far lower energy. Fundamentally, it doesn't really do anything that the same amount of heat doesn't. (and the amount of energy radiated by a cell phone is minuscule) But it doesn't work in _exactly_ the same way, so you can't categorically exclude some obscure, indirect mechanism. But as far as I know, no detailed mechanism has ever been proposed, and there's no empirical evidence to back it up. So in short, it's not known to cause cancer and there's currently no reason to believe it would.
Do small animals get concussions (e.g. insects, small reptiles)?	Hard to say with certainty because we’re only just now understanding specific diagnostic measures and imaging for concussions in people beyond symptom reports and non-specific clinical tests. We would possibly need to screen animals with a fMRI to know conclusively. I’m not familiar enough with veterinary medicine to know if you would see any visual disturbances like you see in people such as nystagmus.
Why do acidic beverages (like soda) make the burning sensation of spicy foods worse?	Capsaicin activates TRPV1 ion channels to give you the burning feelin. TRPV1 channels are heat sensitive but also acid sensitive, so the acid in your drink might directly amplify the activation of TRPV1, thereby bringing about more burning sensation. More likely and physiologically relevant, however, is perhaps an indirect mechanism. Lowering of the pain threshold by TRPV1 activation by capsaicin, facilitates easier pain transmission following the activation of other acid sensing ion channels. So, the capsaicin, which has already lowered the pain threshold, is therefore allowing acid sensing neurons to more easily cross the activation threshold required to sense pain at a certain acid concentration.
Why do balls curve when spinning through the air?	I think you're talking about the [Magnus Effect](_URL_0_). In simple terms, when an object is spinning as it moves through the air, one side is moving with the air and one side is moving against the air. This leads to a higher pressure on side compared to the other, and just like an airplane's wing, this acts as a lift force perpendicular to the ball's motion.
Is the expansion of the universe accelerating?	Just a side note. Don't think of distant galaxies as moving through space, because they aren't. Think instead of changes in geometry over time, because that's what's happening. When we talk about accelerated expansion, we're talking about the way the rate of change in geometry changes with time. The essence of it is that the distances between fixed points in the universe are increasing over time. Take any two points, measure the distance between them, then wait a reasonable amount of time — say a dozen billion years. Measure the distance again and you'll find that the distance has increased. The two points are not moving. But the distance between them is not fixed. So when viewed from a single point at a single instant, it appears that objects sitting out in space at those fixed points are receding from us, and that their speed of recession is proportional to how far away they are. But we know that isn't the case. It's just an optical illusion.
When we close our eyes, is the blackness we see the back of our eyelids? If so, do blind people 'see' the same blackness that we do?	When I was a lot younger my class went on a field trip to a seeing eye dog kennel. There was a blind lady there who talked to us and then took our questions. Someone asked what she saw as a blind person. Her reply was "You all know what blind people see, you just don't see it. Look through the back of your head and you will see what I see." Since we were so young I don't think anyone, including myself, knew what she meant. Then years later it dawned on me that our vision only spans 180deg. We are blind in the other 180 (behind us).
If nothing can travel faster than the speed of light in a vacuum, why have I heard that the expansion of the universe will eventually move faster than light?	The short version is that while no object's position can change at a rate faster than c, there is no such restriction on how fast the distance between us and some distant object can increase. This is often said as "nothing can move through space faster than the speed of light, but space itself can expand faster than the speed of light". For the long version, see my comment [here](_URL_0_). Also, it isn't really correct to say expansion will "eventually move faster than light". Expansion doesn't have a speed, as such, because it causes things that are further apart to recede faster. An object one megaparsec away recedes at 71 km/s, an object two megaparsecs away recedes at 142 km/s, and so on. Thus, objects sufficiently far away are already receding at speeds greater than the speed of light (and some have been doing so since at least the end of inflation about a trillionth of a trillionth of a trillionth of a second after the initial big bang singularity ).
Why do dogs get cancer after only 10-15 years when humans take many decades to develop most cancers.	Because cancer is, on a large scale, an age-related disease, or rather a cell-age related disease. Dogs age differently than humans. So let's say that some of their tissues regenerate faster, the cells within the tissue will have a more dynamic cell cycle thus augmenting the chances for mutations on oncogenes or proto-oncogenes. Aging is a mechanism that's supposed to restrain malign cells to develop, faster aging kinda links to faster cancer development. I can go into more detail if that's not clear enough, but this is the big picture as far as I know. Of course, dogs are still subjects to environmental carcinogen challenges, but even these kind of cancer are inherently linked to dynamic tissus that would accumulate these said carcinogens Edit: trying to get my english spelling right
Do 15 year old dogs get cancer at rates similar to 15 year old humans, or to humans at a similar point on their lifespan?	Definitely the latter . A 15 year old human is an adolescent. Cancer in adolescents (14-19) is pretty rare , accounting for less than 1% of all cancers. They also tend to be different types of cancer than what children or adults get. This would be more equivalent to a 1-2 year old dog. A 15 year old dog is beyond geriatric. That's beyond the normal lifespan of many breeds. A better example might be comparing to a middle aged or older dog (10-12) in many breeds. Their rates of cancer are going to be higher, comparable to people above 65. . Age is a big risk factor for cancer; certain cancer types in particular are linked to aging.
Why do all the planets orbit counter clockwise?	They look like they're going clockwise if you look from one direction, and counterclockwise if you look from another direction. If you don't believe me, find a glass clock and look at it from behind. I'm getting downvoted so maybe I should clarify: Clockwise and counterclockwise are terms relative to some axis. For a clock on a wall, it's the axis coming out of the wall. For the Earth, it's the North axis. If you reverse that axis, clockwise becomes counterclockwise. A clock from behind looks counterclockwise, the Earth viewed from the South looks like it's rotating clockwise. Because there is no definite up or down in space, there is also no clockwise or counterclockwise. The planets all rotate in some direction, and that is counterclockwise if the sun's North is considered up.
What happens within your body for an itch to occur?	Often there is an “irritation” occurring, for instance blocked pores. The act of stimulating multiple adjacent nerves of the skin (itching) causes the nerves to enter into a “numb” state. This is why scabs often itch, or areas where sweat gets trapped
Will Pluto ever collide with Neptune?	Not in the foreseeable future, because Pluto is in a 2:3 orbital resonance with Neptune, meaning that Neptune orbits the Sun three times for every 2 orbits that Pluto completes. This resonance keeps them fairly far apart at all times despite having orbital paths that come close. However, the Solar System is ultimately chaotic, meaning that the gravitational influences of the various planets have the potential to perturb any equilibrium over time. It's possible (although highly unlikely) that Pluto's orbit would get perturbed in such a way that it would collide with Neptune.
Can Pluto collide with Neptune in the future?	Diagrams of the Solar System give the impression that the orbits of Neptune and Pluto cut right across each other, and textbooks even state that Pluto crossed the orbit of Neptune in February 1999. Yet in reality the two planets can never get close to colliding, for two reasons. Firstly, the apparent crossing-points are optical illusions, caused by the fact that the two orbits are actually steeply inclined to one another. The second reason that Neptune and Pluto can't collide is because Neptune's orbital period of 164.8 years means it makes three orbits for every two made by Pluto, with its 248.8-year orbital period. That puts them in a so-called gravitational resonance, where each planet speeds up or slows down as the other approaches, which alters their paths and prevents them coming closer than around 2600 million km to each other. source : _URL_0_
Why is the ozone hole only over Antarctica?	In short it's that the conditions there are most suitable for ozone depletion. The polar vortex isolates the Antarctic atmosphere in winter, and the extreme cold allows the formation of high altitude ice clouds, and the ice provides a platform for the damaging chlorine molecules to deplete ozone. An atmospheric scientist could provide a more comprehensive answer, but that's the basics of it.
If carbon dioxide is heavier than oxygen and nitrogen then how come it rises in the atmosphere and causes the greenhouse effect? Not trying to be controversial, I just don't understand it.	If you have a large amount of carbon dioxide in one place, it will sink in air as the bulk density of CO2 is greater than that of the surrounding air. This only holds true for as long as the pocket of CO2 remains undiluted. With smaller amounts, the density difference is overcome by the kinetic energy imparted by random collisions with neighbouring gas molecules, such that the gases mix evenly. This is why we can breathe air, as the oxygen and nitrogen have different densities but do not stratify in the atmosphere. CO2 currently comprises about 400 parts per million in atmospheric air.
Is The Universe expanding uniformly?	There's no signal telling the Universe to expand. The Universe is expanding because it was at some point - we don't know why - and due to inertia, galaxies continue moving away from each other. It's much like if I throw a ball in the air. Once I've thrown it, there's no signal telling it to keep going up. Inertia does the job. And yes, as far as we can tell, this is completely uniform on large scales.
Why does lightning occur when a volcano erupts?	Volcano lightning hasn’t actually been studied in much detail. I believe it’s the same mechanism that creates lightning in normal storm clouds: particles of solid matter rubbing against each other creating electrical charge differentials between two different parts of the cloud. When the differential gets large enough, there is a spark between them to correct the imbalance.
How truly accurate is carbon dating and how does it work?	Carbon dating is very accurate. There are two kinds of carbon that occur in large amounts C-12 and C-14, C-14 decays. When a plant is alive it takes in carbon dioxide and uses that to make its structures, in this living plant we would find C-14 and C-12 in ratios very similar to those that occur in the air. When the plant dies it stops res pirating, thus no more C-12 or C-14 are put into the plant. The C-14 slowly decays (at a known and constant rate) into C-12, so as the dead plant matter goes on in time the C-14 becomes less than in the atmosphere and the C-12 becomes more until there is no more C-14. By taking a sample of old plant matter (wood for instance) and looking at its ratios of C-14 to C-12 you can get an accurate date for its age.
What happens to the insects after I spray them with Raid or other bug spray?	Bug spray is a low-level nerve gas (please be careful!) that affects the insects just like pepper spray, mustard gas and cyanide would do to us (three different levels, right there-Google each one). The bugs nervous system is shutting down, the air that surrounds it becomes toxic and death follows painfully. But, it is a bug that wouldn't think twice about biting you, stinging you or landing in your potatoe salad when you're trying to eat, sooooo, fu*k 'em! Spray away!
Maybe I'm just misunderstanding how telescopes work, but how is it that we can see things on our own galaxy as if we were thousands of light years away?	Well, first of all, that isn't a photograph. It is more along the lines of "an artist's conception." Secondly, we can obtain information of structures like this one through a plethora of techniques like using different wavelengths for telescopes (those specific lobes were viewed with a gamma ray telescope) or gravitational lensing of other galaxies. All of our galactic observations are taken from essentially the same place. We do have distant probes like Voyager and New Horizons, but they are still essentially in the same spot we are.
Why don't electric/hybrid cars incorporate photovoltaic panels into their roof/hood/trunk?	I don't work in the auto industry, so I'm not an authoritative source. From what I understand, it's essentially because solar panels are not currently efficient enough to provide any real charging benefit. Suppose you were able to get even 200w from a roof mounted solar panel. The Chevy Volt has a 16kwh (16,000wh) battery that lasts about 30 miles. Assuming 100% charging efficiency, you'd get 0.375 miles per hour of peak sunlight. That's what? Assuming you get 1,200wh/day, that's 2.25 miles sunlight powered per day? You'd have to wait two weeks for a full charge? Not to mention the added cost/complexity.
How is #5 plastic made, and what is it that makes it better then other types for heating food in the microwave?	Number 5 plastic is [polypropylene!](_URL_0_) By the nature of its chemical bonds is very thermostable, and it does not tend to weaken under high heat. Other plastics are more prone to decomposition at high heat and with thus melt and are known at thermoplastics. Hopefully a chemist or polymer science person will arrive soon to help you!
How long would it take to "cook" pasta using water at room temperature ?	> Cooking dried pasta is just re-hydrating them. That's not the whole story. While the first step of cooking dried pasta is to rehydrate them, there is an additional step that occurs in both dried and fresh pasta - [starch gelatinization](_URL_0_). Normally amylopectin in starch is highly branched and hydrogen-bonded to itself/each other in a stiff crystalline structure. When heated those hydrogen bonds loosen and water is allowed to hydrogen-bond, making your starch a softer gel. This can only occur when sufficient heat is applied.
Why do we need to boil pasta? Why isn't it enough to soak it in water?	You need the heat from the boiling to cook the starch/flour which is an ingredient of the pasta. Try this at home. Take some flour and mix it with cold water. do the same with boiling water. you'll notice the difference. only by cooking the pasta gets soft but still sticks together.
If different breeds of dogs have distinct personality traits, then why too can't different races of humans?	Not exactly, because dog breeds are the result of artificial selection. The reason some dog breeds have distinctive personalities is because we bred them to be that way. In this respect, dog breeds are an extremely poor model for human races.
Are the differences between breeds of dogs similar to the differences between ethnicities of humans?	The human species isn't exactly outbred - the [effective population size](_URL_1_) is on the order of 10,000 (meaning that our several billion people, breeding nonrandomly due to geographic and sometimes cultural constraints, have roughly the same genetic diversity as only 10,000 people breeding in perfect randomness), but domesticated dogs are extremely inbred: according to [this paper](_URL_2_) most breeds' effective population sizes are on the order of 40 to 80. That's why a lot of breeds have high incidence of genetic disease: the lack of [heterozygosity](_URL_0_) exposes them to harmful [recessive](_URL_3_) diseases that wouldn't be very common in a more diverse population of the same size. But that's a difference of degree. Sure, if humans went through a genetic bottleneck as tight as the ones imposed by dog breeders, or selection that strong, we could diverge into very distinct breeds too, and enjoy the same heightened risks of recessive disease. We haven't yet.
Why don't dogs & cats need eye glasses?	If your dog/cat was slightly shortsighted or had astigmatism, how would it tell you? Cats and dogs rely more on their sense of smell than humans do, and they have better hearing, so any defect in sight would be offset by this more so than in humans. Dogs and cats do sometimes lose their sight, get weak sight, or go blind. The owner just has to let it happen, as cats and dogs wouldn't enjoy or tolerate contact lenses or eyeglasses.
Is the universe constantly getting warmer by the heat generated from stars and planets?	No. From what I understand the universe is cooling because it is expanding. Astronomers have taken the temperature of the universe and found that it has cooled as the Big Band theory predicts. Its about 2.73 degrees Kelvin. That is -270.27 degrees Celsius. They did this by measuring the temperature of gas in a galaxy 7.2 billion lightyears away. The cosmic background radiation is the only thing keeping this gas warm. _URL_0_
Why is torque a vector? Right hand rule is a bit arbitrary isn't it? What does the direction of the torque vector signify?	Torque causes an object to rotate, and rotation in 3 dimensions is determined by an axis, the direction of rotation about that axis, and an angle. The torque vector direction determines the axis and the direction of rotation about that axis. (Since we are talking about continuous rotations, the angle part isn't really given by anything, but the magnitude of the torque is used to determine the rate of rotation.) If you point your right hand thumb along the direction of the torque vector, then your fingers will curl in the direction of the rotation induced by that torque.
Normal stars are made out of hydrogen which undergoes fusion. Can I make a star out of any element, provided I have enough mass to fuse that element? Say, Iron?	Only elements to left of the peak, which is actually Iron, your example, can be fused to yield energy.: _URL_1_ Stuff to the right can be fissioned to yield energy. So things as heavy or heavier than iron signal the death of fusion in a star, as fusion then eats up energy rather than yielding it.
Why does organic milk take longer to expire than a non-organic store brand?	But does it? [This article](_URL_0_) points out that it's not because it's organic, but because it is treated differently than normal milk. UHT pasteurization modifies the taste of the milk, so I prefer normal milk, because of taste. Furthermore, in norway, the organic and non-organic milk (which is both pasteurized at same temperatures as far as I know) show approximately the same date as expiry.
How come it seems that the majority of dna mutations result in cancer of some type?	Mutations happen all the time, but they mostly will harm the cell enough for it to just die off (and you never see it). Mutations that predispose to cancer can be of various kinds. Some of them are: Mutations in genes that keep cells alive (so the cells are kept alive even if they should die), mutations in genes that kill cells (so the cells aren't killed even if they should be), mutations in error-correction genes (so the random errors that happen in DNA replication don't get fixed and tend to add up), mutations that make the chromosomes unstable, and so on. Even mutations in genes that control how easily (or not) a cancerous cell can move around the body (=metastasis) could affect your risk of getting a (metastatic) cancer. So cancer is really what happens when a cell does not get signals (or ignores them) to stop growing/to die when it is supposed to, or gets too many "stay alive" signals. The actual mutation can be in a lot of different parts of those pathways.
Can stem cells replace brain cells?	yes you actually have a small subset of cells in an area of your hippocampus called the dentate gyrus that has stem cells that undergo neurogenesis and form new brain cells. As for implanting stem cells and having them form brain cells this is also done relatively easily! The only problem is forming correct connections and not causing unwanted growth, which is incredibly difficult to do, and in a healthy individual there should be no reason for this.
Is it likely that stem cells will be able to restore cognitive abilities in brain-damaged individuals at some point in the future?	Females that have had children have shown to possess small amounts of healthy working brain cells as a byproduct of the pregnancy itself, these neurons having male DNA if the child was in fact male. This means that new neurons can work with old systems, HOWEVER a noticeable (not insignificant) change in cognitive ability in either direction requires an enormous amount of damage (or repair), unless said damage (or repair) is in small specific zones like say, Wernicke's area; a small amount of stem cells could restore a significant (in appearence) amount of damage. Damage from ischemia or massive head traumas would physically require an unthinkable amount of working new neurons and synapses
Are we rotating around the sun or just being dragged through space by the sun in an elliptical-type motion?	The earth is absolutely rotating around the sun. When we talk about the solar system we usually imagine a reference frame where the sun is at rest. The earth will not end up trailing behind the sun linearly as it has nowhere to lose all it's angular momentum to. Rather than trailing behind it the earth would fall in to the sun if you removed it's orbital motion, just like a satellite in orbit around the earth does when slowed down sufficiently.
If you're travelling towards a rotating planet (at near light speed or not), does that planet appear to rotate faster?	Think about this: suppose the planet was coupled to a giant antenna which gave off an electromagnetic wave that peaked every time the planet completes a full rotation. In other words, the frequency of the wave is the same as the frequency of the planet's rotation. Since you're moving relative to the antenna, the wave would be subject to the [relativistic Doppler effect](_URL_0_). So the planet's rotation would have to appear different by the same factor. Thus, if you're moving toward the planet, you see it rotating faster, and if you're moving away, you see it rotating slower.
What is the scientific/ biological explanation behind female menstrual cycles syncing?	Menstrual synchrony seems to be just plain chance, as far as I know. Menstrual cycles of women greatly vary, and supposed synchrony is usually noticed with a wide range. Given a group of women whose cycles can have a difference of two weeks, and which last only a few days to over a week, they'll eventually occur at roughly the same time, before moving apart again. Maybe there's something else to it and it really does exist, but evidence is slim and research pretty inconclusive. Probability seems to provide the best answer.
Why are biofuels considered less "powerful" than regular fuels?	The common biofuels (ethanol and biodiesel) have lower energy densities than conventional fuels -- i.e. they release less heat per kg, when burned, than conventional fuels. This is simply related to their chemistry -- they tend to have carbon-oxygen bonds as part of their structure already (the alcohol group in ethanol, and the ester group in biodiesel). It isn't inherent to the fact that they come from biological sources -- you could make ethanol from an entirely non-biological source, like natural gas, and it'd still have a lower energy density.
GMO's vs. artificial selection; How are modern GMO's any different than other artificially selected crops?	Key difference is that in artificial selection, you're not dumping in something new(Not already found in that species). GMOs however, allow for us to say, take a gene sequence encoding for a certain product from a fish, and dump it into corn. As for the fear of GMOs, (Warning: Opinion) I feel that its got quite a relation to tradition. Its new, its confusing, and people haven't gotten used to it yet. There are some justifiable fears, I'll admit, but fundamentally, there isn't really an issue with GMOs
Is infinity a valid result of mathematics?	"Infinity" is a very useful concept, but it's not a number. Saying "∞/2=∞" is not arithmetic; it's just a shorthand for saying "If x→∞, then x/2→∞", which in turns means something like "For any N > 0, you can find a M > 0, so that for any x > M, x/2 > N." Also, by playing fast and loose, you can "evaluate" a formula that contains one "∞", but not a formula that contains more than one "∞".
Why is oil hydrophobic?	It all comes down to the polarity of the molecule. Water is polar, which means one side of the molecule is more electronegative than the other. Oil is not. The more positive side of the water molecule likes to be around the more negative side of the water molecule, and neither particularly like to be around the relatively neutral "oil" molecule. This is why oil does not tend to associate with water and is considered hydrophobic.
Why doesn't a space ship continually accelerate?	a spaceship with boosters on that isn't in a significant gravitational field (or one that is big enough to be considered constant) will continue to accelerate, yes. I think the problem you're having is you're imagining sci-fi spaceships. Real spaceships either spend most of their time in orbit with boosters off, or spend most of their time en route with boosters off.
If a space shuttle accelerates in space, what stops it from accelerating with no friction acting on it ?	when you stop applying a force you stop accelerating. that's newton's law. force free motion is uniform motion (no acceleration), not accelerated motion. if you want to keep accelerating you need to add thrust (or for instance use gravitational attraction)
Can humans taste water? Does water have taste? What are we tasting when drinking water?	Pure water, that is H2O, doesn't have a taste that humans can detect. What you taste when you drink water is dissolved solids in the water. Tap water has chlorine and fluoride added to it, plus assorted salts that make it through the purification process. Most bottled water is usually just filtered tap water.
If bacteria cause dead tissue to smell, would a dead bacteria colony start to smell?	Dead tissue smell because its composition is being broken down at the cellular level by bacteria digesting it as nutrients. You can bet that if the bacteria themselves spill their guts after dying, the leaking chemicals and decaying molecules that make up their cytoplasm would start to smell as well.
Why do stimulant-based ADHD medications alleviate many of the focus and "intention" symptoms of ADHD? What exactly is going on that extra norepinephrine and dopamine is able to alleviate these symptoms so effectively?	Dopamine is often thought of in mainstream culture as the "reward" neurotransmitter, but that's a bit of a misconception. It is better characterized as the "motivation" neurotransmitter. You increase the duration of time that dopamine lingers in the synapse (this is what ADHD meds do), you increase motivation. Dopamine makes people want to work for rewards, and technically it does reward them for doing so, but the pleasure obtained from work towards a goal is motivating; so, dopamine is all about motivation. Heroin, by contrast, also creates pleasurable sensations, but it doesn't improve motivation. [If you'd like to read more](_URL_0_)
What prevents woodpeckers from getting concussions?	Oh this is one I can answer! Woodpeckers have lots of neat adaptations that keep them from injuring themselves when the peck wood. One is that they have fairly thick spongy skull bones, another is cartilage padding. They also have a tongue so long it wraps around the back of their skull! Specifically to avoid a concussion, there is no space around their brain that would allow it to move and hit the inside of the skull and cause one. There's a great article on woodpeckers here: _URL_0_ (I'm not with Northern Woodlands, but I love their articles. I'm an environmental educator at a nature Preserve)
Why do humans mature so much more slowly than other animals?	> Could it be related to our intelligence? Does growing slowly allow us to develop richer and more advanced minds? Yes. This is exactly the reason. Being born with a highly "plastic" brain allows us to adapt to the environment and learn from it. This allows us to develop skills (language, tool use, novel problem-solving abilities etc.) as necessary. Neanderthals on the other hand, were born with very "fixed brains". Their brains reached a mature stage much sooner than Homo sapiens, but this meant that their skills were INNATE, and they would have a very difficult time developing new skills (a large reason they didn't develop the extensive language and tool-construction/use that Homo sapiens developed).
If an irrational number has an infinite random sequence contained in it, will it contain another irrational number?	> If an irrational number has an infinite random sequence contained in it, will it contain another irrational number? It will "contain" infinitely many of them, but not in an interesting way. For example, pi "contains" (pi - 3)\10, (pi - 3.14)\10^(3), and so on, and any positive power of 10 multiple of those numbers, all of which will be irrational. It also contains a few with negative power of 10 multiples corresponding to places where strings of 0 appear. > If so, then could one write an expression [pi=k+ex10^(c)] where k is a rational constant and c is an integer? This would just be saying that e is a number of the above form.
Why is salt in food often credited as sodium and not NaCl?	Because not all of the sodium in your food is in the form of NaCl. It's also in monosodium glutemate (C5H8NO4Na), sodium bicarbonate (NaHCO3), sodium nitrate (NaNO3), etc.
If the expansion of the universe is pushing galaxies apart, why isn't it also pushing stars within galaxies away from each other? (Or is it?)	to tack on to the other answers, it's not just that gravitation dominates on galactic scales and smaller, but that expansion of the universe doesn't happen at all on those scales. Only on very very large scales, where the mass density of the universe drops below a critical value will metric expansion be the solution to General Relativity
If I shoot a laser beam into a room of mirrors, would the light forever bounce in that room?	As many people have mentioned, this would only work if your mirrors were "perfect" mirrors, and no mirror can be 100% efficient. However, there is a way you could do this. Build a room out of a material with a high refractive index, and shine a laser beam at an angle great enough that [total internal reflection](_URL_0_) takes place. This is, essentially, a lossless mirror. Now, the room would have to sort of come to a funky arrow shape at either end in order to bounce the laser beam back the other way, but it can be done. p.s- what I described above (minus the bouncing back part) is how fiber optic cables work.
Why is flu season only during cooler months of the year?	Second last paragraph: _URL_0_ > Possible explanations for the seasonality of the flu have been investigated, such as the return of kids to school, people spending more time indoors in the winter, and lower light levels that affect the immune system, but there is no agreement on them, said the NSF CAREER award recipient.
Why is that that "flu season" seems to be in the fall/winter months? Is the virus less communicable throughout the spring and summer?	It seems that the influenza virus survives longer in cold, dry temperatures. Dry air creates lighter mucus particles which travel farther in the air. [_URL_0_](_URL_0_)
Why do we have such effective antibiotics but no true equivalent for viruses?	It's mostly metabolic. Bacteria are living cells with DNA and metabolic enzymes. They can live and divide on their own like other one cell organisms. This gives antibiotics a method of attacking the bacteria- interfere with, or "poison", the metabolism and you can kill them. Viruses are packets of DNA that enter the host cells and take over the metabolism- using the host cells to replicate themselves. The can't reproduce on their own. Without a host cell, it is difficult to even call them living organisms. This makes it very difficult to target a virus using antibiotics- interfering with the viral metabolism means interfering with the metabolism of host cells.
When a child get's an organ/limb transplant, will the transplanted organ/limb grow with the child? If so, will the organ/limb grow according to the donor child's body genetic makeup or the recipient's?	Some organ transplants (such as heart transplants) need to be reasonably size-matched between the donor and the recipient. That means that a baby can only receive a heart from another baby - there's no room to stick an adult heart in. In these cases, yes, the organ does grow with the child. When they first started doing heart transplants, they weren't sure if this would be the case or if they would have to re-transplant every few years as the child grew, but it turns out that the heart will grow. The donor heart maintains its own DNA (hence the need for organ recipients to take anti-rejection medication for life), and would grow according to the donor's genetic makeup. That being said, the organ would also be receiving input cues from its environment (i.e., the recipient's body) that may influence how it would grow.
If I let soda water sit in a perfectly smooth glass cup that has no imperfections or uneven surfaces on which bubbles could form, will the soda water still go flat?	Yes. Diffusion will still occur according to Fick's laws which will deplete the CO2 eventually, it's the same way bubbles grow once formed. The only difference will be that the process isn't "accelerated" by the thermodynamic favoring of a dramatically changing surface energy.
Do standard car windows filter UV light? i.e. Can you get a tan while driving on a sunny day with the windows up?	This was [recently discussed](_URL_0_) in this subreddit and /u/VeryLittle gave a [nice answer](_URL_0_crxk809) explaining the difference between UVA and UVB.
Is there such a thing as Microwave-safe plastic?	Yes, there is such a thing. Bisphenol-A is a subject of recent publicity, as it's a xenoestrogen. Which is not a carcinogen but an endocrine disruptor, which is _suspected_ to cause problems in small children. But BPA is used in PVC, epoxies and polycarbonate, none of which are "microwave-safe" plastics in the first place. The most common microwave-safe plastics are polyethylene and polypropylene ([id codes](_URL_0_)), neither of which would normally contain anything that's even suspected of acting as a xenoestrogen, much less considered a health hazard.
If the big bang is not an expansion from any specific point in space...	> Why then should the outwards pushing force of matter have an(y) effect on the speed of which the universe is expanding? That's just a metaphor, really. We talk about effective pressure on the metric, but there's no real force and no real pressure. As for why, it does because it does. It's a fundamental truth of the universe that the metric is dynamic and determined by energy density and so forth.
Could a planet theoretically exist which is much larger than earth but made of less dense materials and therefore still have earth gravity?	Check out Uranus and Neptune, they have 14 and 17 times the mass of the earth respectively, yet only a gravity of 0.88 and 1.1 of earths surface gravity.
What is the carbon footprint of the recycling process?	[This study](_URL_0_) does a really thorough job of determining the net carbon footprint of recycling various materials. Bottom line is that nearly all common recyclables are worth the emissions associated with recycling, with aluminum being the best. Each ton of aluminum recycled reduces overall greenhouse gas emissions by the equivalent of 8 tons of CO2. Garbage trucks and reprocessing plants might seem like they aren't very environmentally friendly, but the mining and processing of virgin materials is even worse.
Aside from aluminum, what percent of American single stream recycling actually gets recycled on average?	Where are you? It really varies city to city. What you can recycle in Indianapolis is different than what you can recycle in San Francisco. Not because No. 2 plastic loses it's recyclibility as it travels, but different municipalities have different capabilities. Your best bet is to contact your local sanitation department, ask for recycling, and then ask them for your local do's and dont's. Good luck! Edit: typo
A lot of questions about sunburns	* The damage is caused by UV radiation, which crosslinks thymine residues in DNA to form thymine dimers. The dimers interfere with the enzymes that read and replicate DNA. While some of the dimers can be repaired, if too many build up too quickly, the cell will 'commit suicide' via apoptosis. * Sunscreen absorbs the UV radiation, preventing it from entering the cells and causing damage. There seems to be some debate about the possibility that sunscreen that is absorbed into the skin will cause indirect DNA damage that can increase the risk for certain types of skin cancer, but I don't know enough to say definitively. * Melanin, the pigment that makes skin dark, absorbs UV radiation and transforms it into heat, which prevents DNA damage. The more melanin you have, the more UV radiation will be absorbed and the less damage your skin will suffer. Melanin production will increase for most people if they are exposed to UV radiation.
Why do stars and the sun produce energy for such a long time?	* Why do they do so for such a long time? Quick answer, the amount of fuel they have to burn! The mass of the sun is: [1.98855±0.00025×10^30 kg](_URL_0_) 1 mole of hydrogen weighs 1.007g 1 mole of Hydrogen contains 6.022×10^23 atoms you can see then that the order of magnitude of the number of hydrogen atoms is incredibly large, even if the sun is just under 74% Hydrogen. In fact there are about 8.9×10^56 free protons in the Sun * Is there something that limits the speed at which fusion occurs in stars? The fusion rate in the core of the sun is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weight of the outer layers, reducing the fusion rate and correcting the perturbation; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the fusion rate and again reverting it to its present level.
If stars are the main proponent for creating the elements, such as iron, then does that mean at the time of the big bang there was simply a giant pool of hydrogen? Or maybe a giant pool of quarks?	At some point not TOO long after the big bang it was a pool of quarks, then subatomic particles, then hydrogen for a white before the first stars formed.
Is our sun orbiting something?	Yes, we orbit the center of the Milky Way Galaxy. As for G-forces, these are caused by acceleration not by speed. For a test you can do at home, accelerate in your car really fast. You'll feel yourself be "pushed" into your seat. Then stay at whatever speed you are at. No matter how fast you are going you won't experience that force anymore.
Why when listening to a recording of my own voice it sounds different to what I think I sound like?	Basically, it's because the sound of yourself that you usually hear when speaking is the sound that come from your larynx and pass to many sinuses in your skull, nasal cavity and many others that make the sound change from what it was generated from your vocal cord in larynx. But when other hear your sound or when you're listening to your recorded voice it doesn't go through those cavity that I mentioned before. Therefore, the difference occurs. Hope this help.
Do things that have similar scents/smells have similar chemical makeup? Or can two distinctly different things with distinctly different chemical makeups smell the same?	Actually, they just have similar shapes but may have completely different compositions. Certain molecular bits (functional groups as they are called) have shapes that cause certain molecules to fit in certain receptors, but molecular bits that are similar in shape and size can also fit in those receptors. The other half of it is your interpretation. The scent of pine needles and lemons is pretty distinct, and not something most people would confuse, even though the molecules (pinene and limonene) are pretty similar in shape and composition.
What has a greater impact on our skin becoming tanned/burnt- UV intensity or duration of time spent in the sun?	The energy deposited in your skin, which is an equal function of both. Your UV Intensity is characterized by the units: I=Energy/(Areatime) To get energy deposited int he skin ( roughly...) we can say: Energy=I Area * time==~~Area~~ ~~time~~ Energy/(~~Area~~~~time~~) Your skin area stays constant while your outside (you're not cutting off parts of your body..right?). So now we can say that that can be taken out of the equation. So now we know that the energy deposited in your skin is roughly a function of: Energy = time intensity. The way I like putting it is that in a UV tanning bed, if you spend 15 minutes in a bed full of 120 Watt intensity bulbs, its equal 12 minutes in a 150 watt bed. 12 * 150=15 * 120 UV Index would have to be converted into UV rays/cm^2 s to use this method correctly.
How do dogs sense earthquakes before they occur?	Actually it's the ULF (Ultra Low Frequency). They feel it about the same time the seismographs do. _URL_0_
What type of images should we expect from the New Horizons flyby of Pluto and its moons?	The best images they will take will be able to resolve features ~100m across according to the mission website: _URL_1_ Here is an image of Callisto (moon of Jupiter) taken at 100m resolution: _URL_0_
How did such large, or little, quantities of Hydrogen, Oxygen, and Carbon, develop in the formation of Earth, but not other planets?	Mars does have abundant oxygen in the form of silicate. Mars and Earth are both too small (low escape velocity at ambient temperature) to retain hydrogen gas (high average velocity) for any geological period of time. They can hold oxygen (lower average velocity), but it is a highly reactive gas that must be replenished. Jupiter got large enough during the initial accretion phase that it was able to amass and hold a huge hydrogen and helium atmosphere.
Why do some flowers (specifically the dandelions on my lawn right now) close up during the night and reopen in the morning?	Different flowers are pollinated by different insects. These insects will have differing patterns of behavior and so some will be nocturnal and some will be diurnal. So the flowers will evolve mechanisms which allow them to gauge the time of day (probably by level of UV, or visible light hitting basic light receptors) and signal to the plant to not waste resources and risk damage to the flower unnecessarily by remaining open with no reproductive gain.
Why do women typically have higher voices than men?	I think it's the other way around: why do men have lower voices than women? Female is the default, the Y chromosome only has 45 genes in it, compared to 20,000 or 30,000 in the entire human genome. [Women find deeper voices in men sexier](_URL_1_), testosterone lengthens a man's vocal chords to produce a lower voice, but that still doesn't explain _why_. According to [this article](_URL_0_), "evidence suggests men evolved deeper voices mainly for intimidating other men rather than attracting women". edit: I found some links which I posted later in this thread disputing the long held notion that female is the default gender.
Why do solids dissolve best in warm water, and gasses dissolve best in cold water?	[This General Chemistry Online page](_URL_0_) was helpful to me on this question: apparently, it depends on the material, not just on whether it's solid or liquid.
Why do supernovae explode outwards and not just collapse altogether into a point?	I think that this is one of the most common misconceptions in astronomy. That black holes eat anything. For a black hole to eat something the thing must arrive at the event horizon of the black hole. Before that point there is nothing special with a black hole. It is just a large mass. The outer parts of a star going supernova is an extreme environment. Things are flying around at very large velocities. The mass of the black hole at this point is small and will not influence this dynamic much. The gas that somehow fall within the event horizon will be eaten. The rest will be thrown away with large velocities.
Are all the stars we can see with naked eyes part of the Milky way galaxy?	Yes. You can see things like the Andromeda galaxy and the Magellanic Clouds with the naked eye, but not individual stars in those objects. Supernova SN 1987a in the Large Magellanic Cloud was naked-eye visible, but a supernova is, of course, a special case. There was a supernova in Andromeda in 1885 (SN 1885a) that was just bright enough that it might have been visible to the naked eye, but there is no record that anyone actually saw it that way. Edit: Typo fixed (thinkgs -- > things)
Why have SSDs not replaced moving disk storage?	The simplest answer is cost per gigabyte and capacity. Flash storage is much more expensive to produce and the capacity of SSDs are still far behind that of HDD. There is also a disconnect in the cost/gb as when go above a 1TB SSD-the architecture becomes very complicated. Currently, you can get a 1TB SSD for about ~$500. Sandisk announced that plan to release their a 4TB 2.5 SSD but it will set you back [$2500](_URL_0_)-it is not yet available so the price is speculation. To put that into perspective you can now purchase a WD red 4Tb spinning drive (enterprise quality) for about ~~$350~~ $200. Finally, the largest HDD capacity is 12TB and cost about $1500. It is speculated that SSDs and HDD will reach price parity by [~2017](_URL_1_) TL:DR, SSDs are replacing spinning disks drives it is just going to take a while.
Is there any instance of an exoplanet orbiting three separate stars, or is this not even possible?	There is [this one](_URL_0_), where the planets orbits one star in a triple system, where the other two non-planet stars are in a much closer orbit.
Could a smaller star get pulled into the gravitational pull of a larger star and be stuck in its orbit much like a planet?	This is actually quite common, there are more binary stars than singular stars. [They can be used to show that the speed of light isn't added to the speed of the star](_URL_0_), because otherwise the light from the far star would catch up to the light from the closer one as they orbit. Generally though they have a more mutual orbit, as a great size asymmetry is less common. Sirius is an example of a star that fits your criterion.
Gas expands when heated. Gas cools when it expands. What gives?	Both are true simultaneously. Suppose you heat up the gas in a balloon instantaneously. Since it is hotter (ie the gas molecules are moving faster) it will exert more pressure than before on the walls of the balloon and make it expand. But as the balloon expands, energy is being taken away from each air molecule that helps make it expand, since the walls of the balloon are moving away from the air molecules that bounce off it (easy to verify at home with a ping pong ball bouncing between a paddle and a table), so that the gas cools off. At a certain point an equilibrium is reached.
How come we don't recognize the utter ridiculousness of our dreams until we wake up? Why don't we realize it while we're asleep?	Hello folks, welcome to AskScience! As a friendly reminder, in line with our community derived [guidelines](_URL_4_), posts must be scientific and free from anecdotes and/or layman speculation! All posts that do not meet these guidelines will be removed. If you have any questions on this or any other AS policies, please message the moderators!
Do plants grow bigger or faster in a 100% carbon dioxide atmosphere?	No. Higher CO2 levels will improve plant growth but plants still need oxygen for basic cellular functions and energy consumption. They produce more oxygen then they consume, that much is true. But they will not thrive in 100% CO2.
Most animals seem to have an instinctive fighting style, what is ours?	Our intelligence. Tool making and fashioning weapons and traps seems to be our best bet.
Why do animals seem to fight in phases?	I'd like to point out that boxers and martial artists frequently do the same thing.
Why did marijuana evolve to contain cannabinoids?	> Do compounds like THC and CBD (Cannabidiol) provide the plant with some sort of advantage? Plants produce a variety of [secondary metabolites](_URL_1_) which take a variety of forms and are used for a diverse set of tasks. Natural cannabis does not look like what you would expect. It is wirey with small flowers expressing little to no cannabinoids. You see, cannabis has been domesticated for psychoactive properties. Through meticulous selection by humans, we have created a plant that produces a large amount of these cannabinoids. The molecules themselves were likely defensive compounds expressed a low levels by the plant. Humans then selected for high levels and variants which produce optimal psychoactive properties. As opposed to cannabinoids which produce less than desirable effects, such as nausea. While this used the tools of natural selection, it was done artificially. For more information, I recommend reading/watching [The Botany of Desire](_URL_0_).
What is happening when our stomach 'growls'?	Related but slightly different: Does anyone know what causes the weird sizzling/bubbly feeling/sound in the back of your neck that almost always coincides with stomach growling and the feeling of being hungry? If the description of that sensation doesn't make sense... Well... This is the best discussion about the feeling I could find: _URL_0_
Why does (most) food taste better when hot/heated up?	You're asking a subjective question. I prefer to eat almost anything cold, personally. I don't like to heat up left over pizza for example, but not because I'm lazy. Cool tomato sauce is sweeter, the cheese sharper, so on. People like different foods. The case here is that you like warmer food, and I think the reason for the preference is likely explained accurately by [pofo](_URL_0_).
How are rockets filled with fuel?	Solid rockets are typically assembled around the fuel. The Space Shuttle boosters were made by casting large blocks of fuel and sliding them inside a booster casing. Most orbital rockets are liquid-fueled, because the liquid-fueled design separates the engineering challenges of controlling the combustion, from those of shaping the combustion chamber. (Solid fueled rockets can't be controlled once lit, and their combustion rate is determined by the cast shape of the interior spaces inside the fuel). Liquid fueled rockets do indeed generally have a filler neck or similar assembly, routed into each tank. The problem with a liquid fueled rocket is how you get the fuel into the combustion chamber -- you have to force it in, against the ongoing pressure of, well, a firing rocket engine. That requires an insanely powerful pump. The turbopump that pumped fuel into the Space Shuttle main engines generated about 70,000 horsepower just to push the fuel into the combustion chamber.

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