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How do doctors know which antibiotic to prescribe?	Doctors know which antibiotics to prescribe because medical lab specialists perform a series of tests (eg. a serology test checks the patient's blood for antibodies against a specific strain of pathogen). Lab workers use complex but linear test schematics to narrow down what the infection could be. These tests are performed based on patient history. Large studies have been performed, and continue to be performed, to figure out what dose of which antibiotic is the most effective for each classified infection. Aiding in drug choice is the fact that microbiologists have a thorough understanding of cellular function in microbes, and how each drug disrupts those functions.
Does dyslexia occur in blind people, for instance when reading braille text?	[_URL_0_](_URL_0_) & #x200B; Why you can be both blind and dyslexic
Does everything orbiting the Sun equate to as much or more mass as the sun itself?	Not even close. The sun is well over 99% the mass of the solar system; it's over 1000 times the mass of Jupiter, which itself is more massive than everything smaller combined.
If space based telescopes cant see planets how will the earth based European Extremely Large Telescope do it?	By being much much bigger. The big advantage of space telescopes is no atmosphere, meaning no seeing (the twinkling of stars) and no atmospheric absorption of light. The big disadvantage is price and size - it is extremely expensive to launch a space telescope, they can only be a few meters across. Land based telescopes on the other hand can easily be made enormous. We have adaptive optics and other systems nowadays to compensate for seeing. This allows us to negate many of the problems of the atmosphere and use enormous land telescopes.
Cheap and expensive toothpaste - is there really a difference	Denist here. I don't are what toothpaste someone uses as long as it has fluoride in it. But watch out for the abrasive toothpastes, you will lose layers of enamel over time and that is what make your teeth strong and white!
Why does colored plastic turn white when bent?	It's called crazing. Polymers have highly ordered backbones but less ordered overall structure. When you bend a polymer, you apply a directional stress. The polymer strands become oriented with respect to one another and the non-backbone bonds fail long before the backbone bonds. Light reflects off the gaps formed by the partial failures in the material. Crazing does not occur in all polymers, as the ability to orient into highly ordered regions is dependant on the overall polymeric structure. Polymers with fewer, simpler, and/or smaller side groups tend to crystallize more easily. Dowling's Mechanical Behavior of Materials covers crazing pretty well.
Why does boiled water taste "stale" compared to water as it is found in nature?	Boiling water releases most of the gas (CO2, etc) dissolved in it. I feel comfortable saying that would affect the taste.
Beyond color, what makes curly (wavy) hair structurally different to straight hair?	When a follicle is asymmetrical, the hair that it produces is oval in shape and tends to curl. When it's symmetrical, the strand that emerges grows round and straight.
Are there any instances of Solar Systems with hundreds or even thousands of planets?	Using the current definition of a planet, we currently hold the record for the largest known planetary system - with 8. [HD 10180](_URL_0_) comes in close with 7, but there are possibly two more planets unconfirmed that would bring it to a total of 9.
Can a metal detector detect a higher amount of iron in our blood?	The answer to your question is yes, but not from food intake. There is a disorder called Hemochromatosis which results in a higher than normal iron content in the body. A normal person has ~4 grams of iron in their body, while someone suffering from Hemochromatosis might have up to 50 grams in their body. These extreme cases have been known to set off standard metal detectors, such as those found in airports. For reference, a 3oz portion of mussels contains ~6mg of iron. You would have to eat thousands of mussels in one sitting before the level of iron in your body would be high enough to set off a detector.
If the a black hole attracts light, doesn't it mean that a photon has mass? And if a photon has mass how come it moves with the speed of light?	This question gets asked a lot. Check out this [answer](_URL_0_) linked to in the [FAQ](_URL_1_)
If pi has an infinite number of digits wouldn't it technically repeat at some point?	The number 0.101001000100001000001..., where the number of zeros increases by one each time will never repeat. Pi doesn't either. A number will eventually repeat exactly when that number is a fraction of whole numbers.
How do you determine the age of a star or star system?	FYI, someone else just asked this in a [thread](_URL_0_) posted at pretty much the same time, and /u/astrocubs gave a pretty good answer, and about this object specifically. EDIT: The correct name!
Could a moon have it's own moon?	"No moons of moons (natural satellites that orbit the natural satellite of another body) are known. In most cases, the tidal effects of the primary would [make such a system unstable](_URL_0_)." You can have trojan objects in a three body system but they aren't in orbit around the number two object.
A fly (or any other insect) is trapped in my car as I drive to a new location (let's say 50 miles away) where it then flies out when I open my door. Is the insect going to continue on as normal in this new location? Or has me moving it to a new unfamiliar spot doomed it in some way?	The only insects whose life would be damaged by moving them a large distance are hive insects like wasps, bees, termites, etc. A solitary insect will behave as normal, assuming it can find food and others of its species to mate with in the new location.
Why does men get "morning wood"?	> The cause of NPT is not known with certainty. Bancroft (2005) hypothesizes that the noradrenergic neurons of the locus ceruleus are inhibitory to penile erection, and that the cessation of their discharge that occurs during REM sleep may allow testosterone-related excitatory actions to manifest as NPT.[2] Evidence supporting the possibility that a full bladder can stimulate an erection has existed for some time and is characterized as a 'reflex erection'. The nerves that control a man’s ability to have a reflex erection are located in the sacral nerves (S2-S4) of the spinal cord.[3] _URL_1_ You're not supposed to ask questions that can be answered with a simple Google search. _URL_0_.
Is it possible that our Sun is orbiting another planet/star?	No, there's nothing to suggest this. There's no nearby stars massive enough to have that effect on the sun. We have a good understanding of all the massive objects within some lightyears of us.
How do windows block UV light but not other wavelengths?	Every material absorbs light on different wavelengths. Atoms and molecules in gas absorb only on very specific wavelengths while solids absorb light over larger bands. UVA has a wavelength of 315–400 nm and UVB 280–315 nm. Then it just so happens to be that glass absorbs light well up to somewhere around where UVA starts which it can't absorb very well.
Do windows block UV light?	Common glass will block almost all of the harmful UV light that reaches the Earth's surface. Most of the UV light that makes it past our atmosphere can be divided into two bands: UVA (400-320nm) and UVB (320-290nm) as [shown here](_URL_0_). Only UVB light has enough energy to cause direct DNA damage to human skin, which results both in tanning and sunburn. Fortunately, even a thin pane of conventional glass (usually consisting of a formulation called [soda lime glass](_URL_2_)) will block most of the incoming UVB light as you can see [from this transmission spectrum](_URL_1_). In other words, as long as you stand behind a piece of glass you may never tan appreciably, but at least you will be pretty safe from sunlight-induced skin damage.
Why is it that water usually puts out fires while the elements in water usually help combustion?	Because the elements do not confer the chemical and physical properties into the chemical compounds that they make up. In other words, it's because they are different chemicals. It's the same reason that table salt, sodium chloride, is neither an extremely reactive metal (sodium) nor a poisonous gas (chlorine).
Are there stars in space "Independent" from Galaxies?	Yes, they do exist. It's hard to say how many there are, since they're hard to find. Stars can be thrown out of galaxies by gravitational interactions with other stars; it's hard to think of a scenario where a star could form in intergalactic space, though.
The capacity of flash drives, hard drives and SD cards keep getting bigger and bigger. Will we ever reach a limit where it's impossible to get them any bigger?	This depends on the medium you're storing them on. With Magnetic disks, as those used by current, standard hard drives: Yes; this is due, in large part, to platter density vs. storage bit orientation. See: _URL_1_ for a fun little video explaining current technology and how it differs from the old style. With flash: Yes; This is a physical limitation to the size of a transistor on a chip. Currently, Hynix has developed a 15nm process for it's NAND flash transistors, but we are rapidly approaching the theoretical wall for the size of the transistor vs. viability holding information. Also, the smaller the transistor, the more they'll fit on a chip, but the hotter the device will get due to the increase in transistors. ( _URL_0_ ; _URL_2_ ) Quantum computing: Still theoretical, but there would be a theoretical limit based upon the number of storage atoms you could fit into a package. Of course, this would be on the exobit scale (as each atom would hold only 1 bit of information), but there would be a limit.
Is there a limit to how much space Flash drives (like SD cards and such) can hold?	Yes, it's usually printed on them. ... Memory space on flash drives depends on how many transistors which can be fitted on an integrated circuit, and that number doubles every 2 years according to [Moore's law](_URL_1_). Currently we can make ICs with a half pitch (half the distance between transistors) of 22 nm. [Some say that we will reach a limit of 5 nm year 2022](_URL_0_). For comparison, silicon atoms have a covalent bond radius of 0.111 nm.
If the sun is constantly losing tons of mass, why do the orbits of the solar system remain relatively unchanged?	Two quick searches gave me these numbers: * 174 trillion tones of mass lost per year due to fusion * 1.989 x 10^27 tonnes of mass The mass lost per year is 1.74 x 10^11 tonnes. The sun weighs 1.98 x 10^27 tonnes. This is 0.00000000000001% of it's mass per year (1 x 10^-16). After a billion years, we have lost a millionth of the sun's mass. Sun's big.
Why do fish have a vertical tail, yet aquatic mammals have a flat tail? What evolutionary advantages, if any, do they have?	It's not always a matter of evolutionary advantage. There's path-dependence to evolution. Most mammalian spines are structured and muscled to flex forward and backward, so having a vertical tail would require both the evolution of such a tail and a "redesign" of the spine and musculature for side-to-side motion. A related question would be: Why did mammals evolve a front-to-back-flexing spine instead of keeping the side-to-side spine of fishes. The answer is that land animals are adapted to running. _URL_0_
Why people lose hair but not beard?	Testosterone gets converted to DHT. DHT is what causes hair loss in people who have male pattern baldness. At the same time DHT is what causes facial hair growth. DHT blocker like finasteride will help prevent onset of MPB but also block facial/pubic hair growth.
When and where did spoken language first emerge in humans?	There is no clear answer to this question. It is assumed that there has been the capacity for language since the first Homo sapiens sapiens, that is, since the first anatomically modern humans. But we don't have any real way to know whether there was a single first language or whether distinct proto-languages might have been spoken in separate populations that would have evolved on their own or in contact with other nearby populations. James Hurford's The Origins of Language: A slim guide is a good, somewhat accessible introduction to the topic, and Salzmann et al.'s Language, Culture and Society has a chapter on it as well (where they say that at least a monogenesis origin is taken to be more likely).
Why does microphone feedback always make a high pitched noise?	Resonant frequency is a property of the physical coil inside the microphone. A microphone that is designed to respond to bass will be more likely to resonate due to a low frequency excitation (although there are always overtones/harmonics). Chances are the microphones you normally encounter are designed for the human voice, and thus resonate on the higher tones (higher resonant frequency). The human voice is not usually bassy at all. Even a good bass singer isn't going to give you the lowest audible frequencies.
Where does microphone feedback come from?	So it is a [feedback](_URL_1_) mechanism. The microphone picks up ambient noise and feeds it through an amplifier to the speakers. Then the speakers reproduce a filtered version of this noise. Typically microphones are more sensitive to [specific high frequencies](_URL_0_), so these frequencies get more amplified when compared to everything else, hence the high pitch feedback. If the sound the microphone picks directly from the speakers is louder than what it feeds initially to them, then there is a positive feedback and the sound amplitude increases exponentially until the system saturates. If the sound is more quiet that what was initially fed, it gets attenuated and everything should be under control. That is why to avoid feedback one should either move the microphone away from the speakers, or decrease the amplifier gain.
Do thrusters work in deep space?	You have mass that is accelerating away from the space ship. For every action there is an equal and opposite reaction... as the thruster allows mass to accelerate in one direction away from the ship, the ship is pushed in the opposite direction. Edit - Imagine you are floating in space, and have a wrench in your hand. If you throw the wrench in one direction, you will accelerate with an equal and opposite reaction and float away from the direction you threw the wrench.
On a microscopic level, what causes mirrors to be reflective?	This is a very complex subject that is beyond my knowledge. But I can direct you to a book which might possibly give some answers in a relatively layman friendly way. "QED: The Strange Theory of Light and Matter" by Richard Feynman is an adaption of some of his lectures on quantum electrodynamics designed for a more general audience.
What makes mirrors reflective?	Almost all surfaces are reflective - that's how we see them. However, most surfaces reflect diffusely, which means that the surface is so uneven that the light hitting it from any direction scatters pretty much randomly. On the other hand, mirrors are made of very smooth metal, which means that it reflects specularly. All the light from one direction will always end up in the same place, so it forms an image [Difference between diffuse and specular reflection](_URL_0_)
Why do some things burn and some things melt?	Burning and melting are two different reactions chemically. Burning happens when the substance reacts with a gas in the air, rapidly oxidizing (oxidation is where a substance loses electrons). Usually, it is being oxidized by oxygen (hence the name). The oxidation reaction produces a lot of light, which is what you see as fire, along with heated soot from the substance flying up into the air. Melting on the other hand is where the molecules in a substance start shaking so violently that they lose structure, like a building collapsing due to a hurricane. We can predict whether a substance will burn or melt at a high temperature based on whether it has a low electronegativity/ionization energy, and loses electrons to oxygen under heat. Without oxygen/some other oxidizing agent, the substance would continue to heat until it melts.
Why do some things burn and some things melt?	Disclaimer: I am a lowly college student so don't expect flawless expertise from me. Burning is a chemical reaction where he substance turns into another substance (normally through combustion/oxidation), whereas melting is a physical process where the substance changes form. Burning will happen in the presence of oxygen with a substance that has a low activation energy for that reaction. If this activation energy is exceeded before the substance reaches its boiling point, it will burn instead of melting. There are almost certainly exceptions or otherwise weird scenarios, but that's the gist, I think. Did that answer your question?
How often do tornadoes hit areas other than North America?	> Tornadoes have been observed on every continent except Antarctica. However, the vast majority of tornadoes in the world occur in the Tornado Alley region of the United States, although they can occur nearly anywhere in North America.[7] They also occasionally occur in south-central and eastern Asia, the Philippines, south east Asia, like Malaysia,[8] northern and east-central South America, Southern Africa, northwestern and southeast Europe, western and southeastern Australia, and New Zealand.[9] Tornadoes can be detected before or as they occur through the use of Pulse-Doppler radar by recognizing patterns in velocity and reflectivity data, such as hook echoes, as well as by the efforts of storm spotters. _URL_0_
Other than North America, do tornadoes happen elsewhere in the world?	Tornadoes are an international phenomenon, besides North America, Europe experiences a large number of tornadoes, with Russia being particularly susceptible to them as a result of its massive size, and conflicting air masses from the Gobi desert and Himalayas, although most tornadoes there are weak and go unreported. This is a trend across Europe of relatively weak tornadoes except for in the areas along the Mediterranean Sea.
Why do we become aroused during REM sleep?	I assume that when you say "aroused" you're talking about physiological arousal, such as penile or clitoral erections, rather than emotional arousal. Seems like the short answer to that is we don't really know, but it may be related to the release of norepinephrine from the locus ceruleus that occurs during REM. - _URL_3_ These periods of arousal may promote the health of the sex organs, but again we don't really know for sure. You may like to look at the following wiki articles: _URL_2_ _URL_0_ _URL_1_
Why do people occasionally wake up aroused?	The phenomenon is called Noctural Penile Tumescence (NPT). This actually [starts in utero and occurs throughout life](_URL_1_). I don't know if we've answered why it happens definitively, but I think some theories are it's a byproduct of the [neurochemical process involved in REM sleep.](_URL_0_) Ninja edits for formatting
Why do many antidepressants cause an increased risk of suicide in younger people?	One hypothesis I know of off-hand suggests that the "lifting of [psychomotor retardation](_URL_1_)" (a symptom of depression) caused by taking SSRIs would enable at-risk individuals to act upon their suicidal thoughts before any SSRI-related improvements in mood ([source](_URL_0_)).
Why don't humans have a mating season?	Many mammals breed opportunistically. The basic rule seems to be, if a breeding g season results in the survival of more offspring, the there will be a breeding season. If not, then not. Humans, other apes, rodents, domestic dogs and cats and many others don't have a season. The animals that do tend to "plan" their births around the arrival of plentiful food and/or good weather.
Why do humans not have a mating season?	Lots of species have mating seasons, but the other great apes do not. In fact it's not unusual for species living in a tropical habitat with a relatively constant climate to breed year-round. So it's not really surprising we don't really have one.
What is actually happening when someone is knocked out?	Biology major here, (also my first post). When your brain gets knocked around after a good hit, different neural pathways can be disturbed. If the neural pathway that connects your reticular activating system (the part of your brain responsible for sleep and consciousness), is disturbed even for just a second you will be “knocked out”. Hope that makes sense.
Is Mars tectonically active like Earth? Or is Earth unique to our solar system in that aspect?	The consensus is "not any more" and that it may not ever have experienced major tectonic activity as you're probably imagining (plate tectonics). There is however, some residual energy left in the lithosphere and faulting and small quakes are thought to occur, these are what the 2016 [Insight](_URL_0_) Mission is hoping to measure.
Why don't electric vehicles come with 5, 6, or 7 speed transmissions?	With the speeds that are usable on public roads combined with the torque and rpm range of an electric motor, you just don't need them. One gear is sufficient for everything from 0-70+ mph Transmissions are used with internal combustion engines because their range of power (efficiency) is fairly narrow. You need a transmission to stay within optimal operating range for the speeds a car operates. That's why performance cars typically have more gears; to better stay within that power band at all speeds.
What are the uses of GM crops outside of human consumption?	[Bioremediation](_URL_0_) is one possible application. Some plants are good at cleaning up or detoxifying certain kinds of pollution. It would be beneficial to use genetic engineering to further enhance their effectiveness/efficiency on an industrial scale. Feed stock is another. Algae based biofuel is a promising avenue for replacing crude oil in the production of petroleum derived materials. There is a lot of room for improvement in current algae cultivation techniques/methods to up their hydrocarbon production. Odds are genetic engineering will be used to achieve that goal. This is in addition to maize based and newer cellulose based ethanol production, both of which could also potentially benefit from genetic modification.
Why is the Twin prime conjecture not obviously true?	a(n)-1 and a(n)+1 are not necessarily prime. The only restriction is that their prime factors do not include p(1),p(2),...,p(n). For instance, * 2\3\5\7\11\13\17\19 + 1 = 347 \ 27953 * 2\3\5\7\11\13\17\19 - 1 = 53 \ 197 \* 929 This way, of multiplying some primes and adding one, is not a way to make a new prime, but a way to create a number whose prime factors are different than the ones you started with.
Does bees' honey concentrate or filter out toxins from the original flowers?	I don't know about "concentrate" but toxins from the environment are certainly found in honey. Note this doesn't just relate to the bees' ability to filter toxins from nectar before producing honey - it also relates to the flowers' ability to filter toxins from water and soil before producing nectar. Specifically, honey has been found to contain pesticides as well as heavy metal contaminants. In the case of pesticides, the concentrations have always been lower than stated safety limits, though heavy metal concentrations occasionally peak over recommended safety limits. _URL_0_ _URL_1_ I'm not sure though, whether there is evidence of filtering vs. concentration vs. nothing with regard to honey or nectar production. That is, does the honey contain more or less of any toxin than the nectar did? Did the nectar contain more or less of the toxin than the ground water did? I dunno.
When children need an organ donor, must the donor be of a similar size to them or can an adult donate? Will the organ continue to grow as the child grows?	In cases of hearts, a child needs a child sized heart or needs to be at least 5 years old to receive an adult sized heart. Lungs, I believe it's the same as hearts Liver- child can receive half of one in a living donor transplant or a cadavarean donor liver can be split and transplanted into two children. Kidneys- infants need infant kidneys or need to reach age 3-5 I believe to take an adult kidney. I'm unsure about pancreai or intestinal transplantation in pediatrics. I myself am a living donor liver transplant recipient. 7th liverversary is in September!
How is it that when nothing can travel faster than the speed of light, the Observable Universe is over 90billion light years across while the age of the Universe is 13billion years old?	I recommend you check out the Cosmology section of our [FAQ](_URL_0_) which covers all the questions you ask and more.
Would it be possible for a planet have a stable orbit around a binary star system, and remain within a habitable zone?	Yes! But it's a little more complicated than. There are two types of habitable zones around a binary star. One is the normal temperature one that we're used to, but there's also a tidal habitable zone, inside which the varying tidal force is too extreme for habitability.
Why is it easier to keep a bicycle from falling over when riding at a fast speed?	This has come up a few times before. Here's one of the best answers: _URL_0_
Does Earth's oil have a planetary purpose (e.g. tectonic lubrication)?	Oil doesn't exist because the earth decide that the tectonic plates need lubrication, or because an animal had some use for them. In fact, nothing exists for any purpose. Everything in this world is the result of physics and chemistry taking their courses and what happens, happens. Even we and everything we do are nature running it's course. However, there are plenty of bacteria that feed on oil that has leaked into the ocean, and I would bet all sorts of something feed on those bacteria. I'm sure someone else can come up with some other form of life in nature that depends directly on oil.
Why is the Serotonin System the Primary Target for Depression Over Other NT Systems?	I know there are other drugs that target other neurotransmitters, though the serotonin ones certainly do seem the most popular. They have never helped my depression, while gaba drugs have done wonders (gabapentin and baclofen). I've looked into why there aren't more gaba drugs on the market. The funding for the study I read was pulled by the pharmaceutical company before they could finish due to inconsistent results.
If the surface of Mars was densely populated with plant life/algae, how long would it take for oxygen levels to reach habitable levels?	Not really my speciality but from what I understand the important thing is in breathing gases is the partial pressure of oxygen. From Wikipedia, a minimum partial pressure of oxygen for breathing is 16 kPa. At sea level on earth it is about 20 kPa. At the top of Mount Everest it is a third of that, or about 7 kPa, and obviously people can survive there for a while but will die if they spend too long a time. If all of Mars' atmosphere was oxygen instead of CO2 then the partial pressure of oxygen is of course the same as the actual pressure which would be 0.6 kPa. So even with all of it converted to oxygen you're not even close to the levels where humans could survive. The atmosphere of Mars is just so thin.
Why are fingerprints an individually unique feature of the human race? Are there other species that exhibit features like this?	Gorillas and chimps also have them. Koalas have fingerprints that are almost indistinguishable from humans. _URL_1_ EDIT- _URL_0_ Thanks sharoz
Why do animal's eyes reflect light in the dark but humans' do not?	this is a reflective membrane in the very back of the eye just behind the retina called the tapetum lucidum and it is meant to enhance the night vision in animals by reflecting the light that missed photoreceptors on the first pass and sending them past the photoreceptors again for a kind of light amplification
Why are auroras on Uranus short lived and not continuous waves throughout the sky?	While I'm not entirely sure how this would relate to the generation of the aurora, the magnetic field of Uranus (and indeed Neptune) are more complicated than just being at a significant angle to the axis. While the magnetic fields of Earth and the gas giants are essential dipolar i.e. like a bar magnet with a single north and south, those of the icy giants have significant quadropole and octopole contributions (as shown in Figure 2 of [this paper](_URL_0_) if you're able to see that), which is believed to be due to where inside the planet the field is generated e.g. in the mantle rather than the core. How exactly that would effect the formation of the aurora is not, AFAIK, something that's been studied.
Is sneezing from an allergic reaction more infectious than sneezing from tickling your nose?	Assuming the "sneezer" is not infected with anything in the respiratory system, neither a sneeze from an allergic reaction nor a sneeze from tickling your nose will be infectious at all. However, if the sneezer does have a respiratory infection, it is possible that a sneeze from an allergic reaction will be more infectious, because more mucous is likely to be involved and is thus more able to carry out the infectious particles (virus or bacteria) in higher quantities where they can go on to infect the next person. This is all pure speculation because lots of assumptions have to be made here. But from a completely healthy person, neither types of sneezes will be infectious.
What exactly is happening within a computer when a program is "not responding"?	Windows applications run an [event loop](_URL_0_), in which user interaction, windowing operations and various other things get handled. For example, a `WM_PAINT` event informs the application that it needs to redraw its window, which is why you often see [graphical corruption](_URL_1_) on hung/crashed application windows. A "not responding" process is one in which the event loop hasn't been run in a while, which can be for all sorts of reasons - perhaps it's [deadlocked](_URL_2_), maybe it's stuck in an infinite loop due to some logic error, or maybe it's just busy doing work and hasn't been designed for responsiveness.
What causes lightning flashes in the sky with the absence of thunder?	They're so far away that you can't hear the thunder.
Would Earth be able to maintain its orbit if the planets closer to the sun and further from the sun disappeared?	Yes. It would do so even better without Jupiter's perturbation.
Why are younger leaves of some plants a shade of lighter green than the older leaves?	The older leaves contain the full assortment of pigments and have fully mature cell walls. > why aren't the newer leaves, which are usually at the top of the plant, thus getting more sunlight, more enriched in chlorophyll than the older, lower leaves? It's actually the opposite: shade leaves (those more likely to be present on older sections of the tree) are darker green because they have more chlorophyll. Shade leaves need to be more efficient at capturing light than sun leaves, so they need more chlorophyll, not less.
When it is cold enough outside to see my breath, why can I not blow smoke rings?	This is anecdotal but I've definitely blown steam rings in the cold, was just doing it yesterday. Its harder and fainter but doable.
What happens to a disc as I try to spin it near the speed of light?	This is called [Ehrenfest's paradox](_URL_0_) and leads to the curious situation where the radius of the disk stays the same and the circumference shrinks. * R = constant * C = 2\pi\R -- > shrinks Why this happens is that spinning frames exhibit non-euclidean geometry. C/2R != pi. While an odd effect, this doesn't stop you from continuing to accelerate your disk, if you keep trying to accelerate the disk, the resulting centrifugal force is going to eventually overcome the binding force of the disk and it'll explode--this is because true rigidity doesn't exist.
What is the cause of the rumbling sensation and noise when my stomach "growls" from hunger?	Probably find some answers here! _URL_2_ _URL_6_ _URL_7_ _URL_1_ _URL_8_ _URL_4_ _URL_5_ _URL_3_ _URL_0_
sqrt(2) squared creates an rational number from an irrational number; can something similar be done for pi? Are there any exponents of pi that are rational?	No non-zero rational exponent will do this, because pi is transcendental. sqrt(2) is irrational but algebraic - that means there is a polynomial with rational coefficients to which sqrt(2) is the solution. Of course, the polynomial is x^2 - 2 = 0. But pi is transcendental, which means it's not algebraic. So there is no polynomial x^N - R = 0 for any rational R and any integer power N > 0 to which pi is a solution. Ergo, pi^N != R for all rationals R and all integers N > 0. But there is a result that any algebraic function of one variable, applied to a transcendental number, yields a transcendental result. So 1/pi is also transcendental, which means it's also impossible when N < 0. By a similar measure, pi^(1/M) is transcendental, so it's also impossible for any rational exponent N/M. However, there are certainly irrational (and possibly transcendental) exponents of pi which are rational, given by log(rational)/log(pi) = exponent. [ObWikipedia](_URL_0_)
Why do nuclear bombs produce mushroom clouds wile more conventional bombs do not?	Conventional bombs do produce mushroom clouds if large enough. Most conventional explosions are not big enough but many of them do produce them.
How did the Big Bang not form a super massive black hole?	The Big Bang singularity isn't the same as a black hole. A black hole is a point in space time with infinite curvature. The Big Bang singularity was the entirety of space time. And the entirety of space time expanded. That being said. We have no working theory of black holes or the Big Bang. We just see that our models asymptotically reach these points. We can't explain what actually happens at these points, though.
Why is my TV brighter when viewed through a mirror?	This is an educated guess but... are you sure you're not dealing with a contrast issue? ...your eyes adjust to the overall amount of light they receive and may be perceiving the TV brighter if the amount of light reflected by your mirror from other sources is less than what you get from the background when you look directly at the TV... that might be the case for instance if the mirror is small and covers a small portion of your field of view, while the background behind the mirror is darker than the rest of your field of view when you look at the TV directly... just guessing of course, but otherwise I don't see how with a (flat!) mirror the TV would look brighter.
How are some people born with blonde hair that changes to black quickly after? Also blue eyes that change to brown	Very basic response: Melanocytes increase their production of melanin as the child ages.
Can things spin in space with force coming from just one point?	You're both right and both wrong. The resulting motion will be composed of a translation and a rotation. How much of each depends on the details. If you're comfortable with conservation of linear and angular momentum, you can approach it as a collision between some impactor and the end of the pole. It could be that your friends are intuitively imagining a force and a torque being applied to the end. This would be the case if the end were clamped to whatever exerted the force, and that thing traveled strictly in a straight line. The clamp would exert a torque to counter that created by the applied force acting about the pole's centre of gravity.
Why is it when I remove dishes from the dishwasher, ceramic/metal/glass is dry and the plastic is covered in water still?	It has to do with the specific heat of each item. Specific heat refers to how much energy it takes to change the temperature of something. Water has a higher specific heat than plastic, but lower than glass and ceramic. So after the dishwasher cycle finishes, the water on what's inside will begin to evaporate. Evaporation leads to the water cooling down, and absorbing some of the heat from what it is covering, plastic has a low specific heat, so a low amount of heat energy is stored, so all the heat is gone before the water is evaporated. The glass and ceramic have much more stored energy, so they will transfer enough heat into the water for it all to dry off.
Is there an organ or body part that cannot be transplanted?	> Can the brain theoretically be transplanted? I think this would more accurately be called a body transplant.
Why is it that whenever I'm single, I desire to be in a relationship. But whenever I'm in a relationship, I desire to be single.	Its a balance between two different human characteristics. The Coolidge effect describes why we feel the need to have multiple partners and want to exit a current relationship. Basically, the thrill of a new partner causes a surge dopamine in the brain. On the other hand, humans are also wired to pair bond, as that helps us raise our intelligent, complicated offspring. Link to an article describing the Coolidge affect. From this article you can also find information about pair bonding: _URL_0_
In the Pale Blue Dot picture, why is the Earth the only relatively bright object? Where is the Sun and other planets and why aren't they visible?	Well, if you'd had the sun in the frame, the earth would be washed out, so obviously, you've got to shoot the earth and not the sun. also, objects in space are really far apart from one another. it's rare that you have two planets in the evening sky close enough to view together in a scope. so to answer your question, other objects in the solar system are not in the frame of the photograph.
If the universe is expanding, isn't all matter/energy in the universe expanding with it?	Space is expanding, but the forces that keep things together are unchanged. Imagine Holding a sitting dog on a leash while the ground between you expands super slowly. Nothings gonna Happen between you two. Now imagine Holding that dog with a Mile-Long leash out of wet toilet paper while the ground expands everywhere at the same speed. More distance means theres a faster "speed" of the ground expanding between you, so the leash is gonna break. This is why Galaxies scatter around while smaller Things Are unaffected.
Why do we not experience image noise while viewing in relative darkness similar to that of cameras when their sensitivity (ISO) is set very high?	I can't answer this directly, as it is way beyond my area of expertise. But I want to point out that your brain is very good at filtering out things. For example, there is a blind spot in your field of vision, but you don't notice it because your brain 'fills in' the missing information. So it is conceivable that this image noise exists, but your brain ignores that information.
Why is one speaker quieter than two speakers of the same volume	Has to do with a somewhat confusing concept called [impedance](_URL_0_). Impedance is like the physical resistance of a speaker to the power output of an amplifier, and it's measured in ohms. Suffice to say that if you attach two speakers to the same amplifier, you lower the total impedance on that amplifier, allowing it to output more power, and hence more volume. Eventually, if you keep adding speakers, you'll drop the impedance down below the impedance rating of the amp, and it'll burn up or more likely shut down automatically.
What are the differences between programming languages? Why have multiple languages in the first place?	> What are the differences between programming languages? Every language can do pretty much everything, but some things are easier in one language and harder in another. > Why have multiple languages in the first place? Same reason there are multiple examples of any other engineering product. Why have a Cessna 172, a Lockheed Martin F-35, a Boeing 747, and a Lockheed SR-71? Each is an airplane, but each has its own purpose. > how do you get from binary to language commands Different language processors: compilers, just-in-time compilers, interpreters take source code in a programming language, and either execute it directly, or transform it into machine language, or transform it into an intermediate bytecode, which is either executed directly or transformed into machine language. > but how can these then be put together to execute commands? Three words: layers of abstraction.
Why are there different programming languages? Are they used in different ways, or is it more of a preference by the creator?	Another issue that has not been mentioned is simply licensing and other issues of control. You may need or want a feature that is not available in a particular language, or use said language in a given platform. Depending on the licensing that may not be possible or the cost for licensing fees may not be approved. This happened not so long ago when Microsoft was doing things with Java that the company that owned the language, Sun Microsystems, did not approve.
If all of the planets in the solar system came from the contents of the same star (supernova), then why are they all so different?	Your initial assumption is wrong - the solar system, including the Sun, wasn't formed from a supernova, but from gases and dust the started to coalesce, perhaps because a nearby supernova exploded, and the shock waves pushed the stuff together. Gravity took over, and small lumps of stuff became bigger and bigger. Because the original materials were not uniformly distributed, the coalescing lumps had different properties. Also, Earth is not the only planet with water. It is believed that Venus once had water, and the evidence for previous water on Mars is very compelling. Also, we don't know for sure that Earth is the only place that life exists (obviously excluding intelligent life). There's good chance we could find microbial life on Mars, Europa (moon of Jupiter) and Enceladus (moon of Saturn).
Is it possible to make hydrophobic windshields?	It can probably easily be applied fairly easily to a windscreen. more importantly: how does it effect other features of a windscreen? UV, surface contact with OEM and aftermarket wipers etc? how does it perform under fast speeds when water's being pushed up the windscreen? Does it degrade? and if so, how expensive is it to replace? how often? etc. consumer nightmare to replace things like this. does it scratch easily (the outside of a modern windscreen is incredibly hard)? How expensive is it? it's probably not worth making it 100% hydrophobic if it effects so many other features of the windscreen when products that are very effective (given the use of wipers) and well proven are available.
Why aren't permanently hydrophobic windshields standard in all vehicles?	Durability is the problem. Windshields take a serious beating over the lifetime of a car, and they aren't replaced barring major or catastrophic failures. Every rock chip that hits the windshield creates a tiny pit which increases surface area. That pit is a great spot for water (and its high surface tension) to adhere to. Any hydrophobic coating originally on the windshield would be destroyed at that spot and you'd be left with the structural material below (a glass/plastic laminate). I think this is why what most people do now is periodically reapply a coating (rain-x, wax) on the glass. If you used a different base material like a pure polymer, instead of a glass/plastic laminate, you'd probably get better hydrophobicity but you'd have to match all the required structural, thermal and optical properties of the existing material. My guess is that no one has found a cost effective replacement option.
How true is it that fruit and vegetables of different colors have different nutritional values?	Some of the nutritional properties of veggies are also the colors. For example, the class of compounds called 'anthocyanins' are generally purple-red to blue-ish purple and encompass blueberries and purple veggies (red cabbage, red onions, etc.). Another class of compounds, the carotenoids (vitamin A precursors) are yellow-orange-red. Green = chlorophyll, not sure if it has nutritional significance in humans though. So you can distinguish colors and nutrition to a degree. However, some other compounds are inherently in vegetable families (like the compound indole-3-carbinol, native to the _brassica_ family like broccoli, cauliflower, kale. Also, alicins are native to garlic. These two exert no color). So to answer your question, it would be 'to a degree'.
Does the colour of a vegetable and fruit depend on vitamins of it?	There is not a direct connection between color and vitamins. What does give color to fruits and vegetables is some pigment molecules that are inside all photosynthetic cells. These substances are in direct correlation with chlorophyll, as they are “covered” by it. That’s why some fruits, like oranges, have green color when they first appear on the tree, but as time passes and the fruit matures, it takes it’s original color , orange. This happens because when the fruit reaches maturity, chlorophylls that used to“wrap” the pigment molecules, start to decompose and the orange color of the pigment finally shows up. A known plant pigment is Carotenoid.
If an ant was the size of a human could it still be able to carry 50 times its weight?	No. Volume increases by a power of 3 per unit length while surface area increase by a power of 2 per unit length.
If an ant was the same size as a tiger, would still be able to lift 20 times its own weight?	No. The problem starts when you scale up the length and all other measurements in equal proportion, then your volume (i.e. mass) grows with the third power of that length, while the strength of your bones and muscles, which depend on their cross section, only grow as the square of that length. An ant the size of an elephant would collapse under its own weight. You just have to look at the body shapes of very small animals, e.g. ants with their long thin legs sticking out sideways and their large abdomens supported by a very narrow waist, to very large ones like elephants, where the legs are very massive and under the body to be able to support its weight.
How do fingerprints regenerate?	Fingerprints are the result of dermal ridges between your dermis and your epidermis (see [dermal papillae](_URL_0_)). Skin growth, on the other hand, occurs as the basal layer of the epidermis (so it's right above the dermis). This means injuries that are superficial enough not to damage the basal layer (e.g., minor scrapes and abrasions) will not affect the fingerprint. However, some cuts can get pretty deep. If you don't take care in stitching your wound up, you can in fact misalign your fingerprints such that the ridges no longer flow continuously across the scar tissue. You can see some examples [here](_URL_1_) (PDF).
Why can almost every inch of skin get tan/sunburn but not your palms?	Your palms don't get sunburned because they do not spend much time exposed to the sun. Due to the nature of our anatomy, there aren't many casual opportunities for your palms to see the the sun (which at its highest intensity is very nearly directly above us). If you decide to sit outside with your palms facing the sun four a couple of hours, you will receive a very painful enlightenment on this topic. I can't answer the question about people of african descent's skin colors.
Are our gas planets in our solar system almost a perfect sphere/ellipsoid?	There are bumps and structures at the highest parts of the atmospheres of Jupiter and Saturn. I dont know much about them but you can think of the bumpyness of clouds when you look down on them from above on Earth and it is somewhat similar for gas giants. Just on a larger scale and different structures due to the dynamics of the flow. The bumps are small in comparison to the whole planet which is approximately an ellipsoid (or oblate spheroid... which is not even perfect due to tidal forces).
What is the difference between things that are poisonous, venomous, and toxic?	A chemical is "toxic" if it can cause damage to a living organism of some kind through chemical means. "Toxic"and "poisonous" basically mean the same thing, really. An animal or plant that is toxic or poisonous has something in its tissues that, if touched or consumed, will damage that which touched or consumed it. Venom, on the other hand, is injected. So spiders, some snakes, the Pacific blue-ringed octopus, etc. are venomous (not poisonous) because they have glands filled with poison, and have a means to deliver it to their prey or something that is attacking them. Usually this is done by biting, where the venom gland is connected to the tooth in such a way that the pressure of the bite will cause venom to run down a channel in the tooth and into the wound. [NOVA](_URL_0_) has an episode about this subject, as well as a nice, clear Q & A page if you need more information.
Why do some things melt, and some things only burn?	Here are some articles that you might find usefull. Basically in order to melt the solid must have a chemical structure such that adding heat won't break down any chemical bonds through oxidation or other decomposition. [Burn, Char, Melt](_URL_0_) [Wood does not melt](_URL_1_)
How do small insects like ants survive a fall from a,comparatively, massive height whilst larger animals can be injured from a relatively tiny fall?	Terminal velocity. A falling object experiences a downward force from gravity that is proportional to its mass, and drag proportional to its speed and cross-sectional area. When the force from gravity and the force from drag balance, the object stops accelerating. Now, assuming a spherical object (which is a favorite when modeling things in physics) with a constant density: volume, and thus mass, increases as the radius cubed, but cross-sectional area increases as the radius squared. So as we reduce the size of the object, mass falls faster than the cross-sectional area, and thus the force of gravity reduces faster than drag. Thus very small objects have quite low terminal velocities, which is why insects can survive long falls (and why raindrops don't kill you). There's more to the equation, like how an exoskeleton is able to remain more rigid as you decrease the size of the insect, but the low terminal velocity is a huge factor.
How do insects survive falling from much greater heights than other animals?	Volume scales at m^3 whereas surface area scales at m^2. Assume both a human and an insect fall and land on their feet An average ant might weigh about .08 grams. An average humans weight about 81,600 grams. Therefore, a human weighs about 1 million times more than an ant. Falling feet first, an ant has a surface area of maybe .00002 meters (.005m long by .001m wide plus legs). Falling spread out, a human has a surface area of maybe 1.5m (1.7m tall by 0.6m wide plus some extra for the limbs A human then has a falling surface area of something like 60,000 ants, but weighs 1million times more. This is known as the square-cube law. Not only does it result in ants having a terminal velocity significantly less than humans, the overall force that must be applied by legs/carapace when landing is much smaller for a given cross-sectional area.
What is the tingle I get down my back when I listen to a song I really like?	It's called a frisson. I can't explain what causes it, but there's a subreddit ( r/frisson) for frisson inducing songs/poems/etc.
What is happening when I get the "shivers" and my skin tingles while listening to a really, really great song?	It can be caused by a dopamine response, and [here's a paper](_URL_0_) describing it being caused by, or at least being contributed to by, the constant denial/payoff expectation reaction in every human being, which can cause "chills" in a variety of situations (just like being scared, or being touched in a sensitive area unexpectedly). Interestingly, you're more likely to get this feeling from music in a minor key (researches often mistakenly call this "sad music", even though that's only partially accurate as well as being misleading terminology). My hypothesis is that this is because minor key chord progressions are simply harder to predict. Source: MFA in music composition and performance and serious nerd.
In flatland, would the 3rd dimension be time?	It could be. However, answer me this. What is the second and third dimension to us? Is it width and height respectively, or length and width? The dimensions are arbitrary named and you would be correct if you named time as the first or second dimension. So yes, if in flatland you declare time a dimension you are wanting to measure, then you can name it the third dimension.
Do prime numbers cease to exist at some point on the number line?	Nope, the number of primes is provably infinite. Here's the oldest proof (over 2000 years old!) that there's always another besides the ones you know about: * Take all prime numbers you know of and multiply them together * Add 1 * Factor the result into primes. The result must include a prime that was not in your original list. There are other proofs as well, this one is just fun because it's so old (in my opinion, anyway).
Why are Neanderthals considered a separate species and not a race of Homo sapiens?	Actually, in some classifications Neanderthals are considered a subspecies of humans. Although it is common for the classifications Homo neanderthalensis and Homo sapiens, there are also those who use the classifications Homo sapiens neanderthalensis and Homo sapiens sapiens. This is a point of contention as some do consider Neanderthals as the same species as humans earning the Homo sapiens name, but others do not strictly classify species as creatures which can interbreed and wouldn't include Neanderthals in their classification. Really, the designation species isn't as clearly defined as we like to think and there can be a significant amount of debate of whether two creatures are members of the same species or not. Still, although I am not sure that you would call them a "race" of humans as they are very morphologically different, there is definitely the camp of evolutionary anthropologists who would classify Neanderthals as Homo sapiens neanderthalensis.
Why are Neanderthals classified as a different species from Homo Sapiens?	Speciation can happen both from inability to have viable offspring and from behaviors or geographic locations that create barriers to having viable offspring. Speciation is also considered a more gradual thing than a lot of people realize. Finally, there is evidence that surviving Neanderthal genes in modern humans are the result of male Neanderthal-female human pairings, one possible explanation for which is infertility of offspring of the opposite pairing.

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