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How is rocket fuel made?
Depends on what rocket fuel you are talking about. Most liquid rocket engine use some form or kerosene or liquid hydrogen. Those are usually made with hydrocarbons like oil and natural gas. However you could use hydrolysis or biofuels to make them. It would just be more expensive. Rockets also traditionally use liquid oxygen that is just produced by liquefying air. The Russian Proton rocket uses UDMH and nitrogen tetroxide (NTO). NTO is made from ammonia which is usually made out of nitrogen and hydrogen. UDMH is a bit more complex (lots of steps if you want to make it from scratch) to make but it's basically ammonia + salt (NaCl) + water.
Why do we have a Lyme Disease vaccine for canines but not for humans?
The primary reason we no longer have a vaccine against lyme disease is contained within the linked article. The vaccine was effective and safe but it fell victim to the circumstances of the time in the aftermath of the Andrew Wakefield MMR travesty. It was killed by the anti-vaccination movement and was no longer economical for the drug company to manufacture.
Why are some people better at singing than others?
Another professional singer here, to sing well takes a lot of practice and dedication. One can have natural talent, have a good ear, etc.. However there is a lot that goes into singing, such as learning to sing on the breath instead of with the breath, learning about adequate space, posture, and certainly musicality. I don't know a lot about the molecular level, but as far as your vocal chords go, everyone has their own unique vocal chords. A soprano's vocal chords will be smaller, thus producing a higher sound, and a baritone's vocal chords will be larger, creating a lower sound. You can read this, it might help you understand a bit more: _URL_0_
Is there any biological reason why some people sing better than others?
There's a difference between singing in tune and sounding good. Singing in tune is a matter of practice. For some people it seems natural, because they've been listening to music and singing since they were a toddler. Practicing unintentionally. Practically anyone can learn to sing in tune given enough practice. Sounding good is a mixture of things as mentioned in other posts here. People can be trained in styles of singing (see: opera) but ultimately the subtleties of their sound come down to physiological factors.
Why is isopropyl alcohol sold in a 91% solution?
According to [wikipedia](_URL_1_), isopropyl alcohol forms an [azeotrope](_URL_0_) with water at ~88%. This means that it is no longer possible to separate the water from the isopropyl alcohol by distillation. You have to use other methods.
Why is isopropyl alcohol sold in drugstores, etc., instead of propyl alcohol?
Isopropyl alcohol is much easier to make - you throw in propene, water and a small amount of acid into a pot and you get the alcohol out. n-propyl alcohol would require some pretty fancy catalysis if done on the industrial scale to get the hydroxyl group attached at the primary carbon. Because it's easier to make, iPrOH is much cheaper. In terms of safety, I think isopropyl alcohol is actually a little bit more dangerous, but both have ethanol-like symptoms of intoxication up until LD50. So to answer your question, I think it's mostly economics.
[Medicine] Do lactose intolerant people gain any nourishment from dairy?
In addition to what DeePhlat said, lactose intolerance isn't an on/off phenomenon, it varies quite a lot by degree. Many with it can eat hard cheese without concern, some can eat any cheese, yogurt, or ice cream without problems. And the response also varies by degree, many people just get bad gas. Those who get diarrhea may get it rapidly or it may take many hours, so the milk product stays in the gut for a while, and things other than the sugars can get fully digested.
If one is lactose intolerant and they regularly consume lactose free milk, is this still potentially bad for them?
Lactose is a disaccharide sugar made of a bound glucose and galactose. During digestion, lactose gets broken up into these two constituents. Generally, someone who is lactose intolerant is simply lacking an enzyme called lactase that breaks that bond. Someone who is *lactose tolerant* is not utilizing intact molecules of lactose anyway. Therefore I can't see any negative effects of cutting out lactose from the diet. Lactose free milk has simply been "predigested" to cleave the lactose into glucose and galactose.
Earth's atmosphere is about 78% nitrogen and 21% oxygen. If the nitrogen was replaced with another nontoxic gas (like helium) would we still be able to breathe it?
Yes - but it depends on the gas. It would need to be something inert so no chemical reactions in the body. Helium-oxygen mix is actually used for deep Scuba as nitrogen becomes narcotic at high pressure. Nitrogen is inert enough at normal atmospheric pressure though.
If all the cells in our body are replaced in x number of years, why are tattoos able to persist?
First of all, much of the body does not undergo turnover. Classically, the brain and the heart are thought of as terminally differentiated, such that damage is often irreparable. With regards to tattooing, remember that tattoo ink pigments are deposited in both the [epidermis and the dermis](_URL_0_). While the epidermis is undergoing constant turnover, the dermis is much more stable. There is no mechanism for 'shedding' from the dermal layer. However, there is some phagocytosis of pigments by macrophages, which is a mechanism laser tattoo removal utilizes.
Why have some plants evolved to be spicy?
Spicy foods are most commonly foods that activate specific receptors (mostly TRPs of the vanilloid family) of pain stimulating nerve fibers, so called nociceptors. What that means is that they cause a pain response in most animals that would eat them or even come in contact (ever tried rubbing chili or any other capsaicin containing food on your skin?); and indeed, spicy taste is not actually detected by specialized taste buds, but by nociceptors of the tongue. This is supposed to prevent animals from eating the plant. However, similar to bitterness (a signal most commonly associated with poison) an individual can get accustomed to the spicy taste. Additionally, some spices also have antibacterial and antioxidative properties.
Why did chili evolved to be spicy?
It's advantageous for a plant to have its fruit eaten by birds. This is because seeds largely pass through birds undigested, and because they spread much farther. On the other hand it's not so good to be eaten by mammals, because they generally kill the seeds and anyway don't travel very far. As it happens, birds can consume very hot peppers without any ill effects.
How large was the moon that formed Saturn's rings before it broke apart?
That the rings originated in a moon is disputed. We don't really know how the rings have formed. There are several theories. - the rings did form when a moon broke up - the rings are just left-overs from the formation of the solar system, they rather never ~~"congealed"~~ coalesced into a moon - anything else in between The current estimate for the total mass of the rings is 3 x 10^19 kg, slightly less than the mass of the moon Mimas, a fairly small satellite. _URL_0_ _URL_1_
How do children who haven't learned to speak yet think? How do animals and small children think without language?
I think they think in impulses. it's hard to wrap your head around now, but imagine this scenario: have you ever done anything without thinking about it? Some kind of task that you've done enough, you could almost do it in your sleep? What is your thought process during those moments? Mostly empty, right? With some impulses? You 'think' without language all the time, but just don't notice because it's so natural. The moment you think back on your thought process, you end up putting it into words but during the actual moment, you may not have thought any words at all. Just impulse. Does that make sense?
Why don't we have satellites orbiting all of the planets in our solar system by now?
So, a point of misunderstanding here is the idea that we haven't been sending things to other planets. _URL_0_ this info graphic is pretty cool, and gives you a sense of how many things we actually have launched. Also, it actually is quite difficult to get things there, and costly. For instance, New Horizons, which is currently on its way to Pluto _URL_1_ , and from the wiki article: "The cost of the mission is approximately 650 million USD over 15 years." So it is quite expensive. And quite exploratory. This is a mainly scientific mission. Also, sadly, the government dosent seem so enthused about funding NASA and future space exploration and travel, which is a major setback for this. Also note, it will take New Horizons 9 years to travel from earth to Pluto, and it is currently the fastest things Humans have ever made. So, that's a problem as well.
Are there any man-made satellites orbiting the sun instead of the Earth?
[Yes, there are many.](_URL_0_) Bonus: Mercury - Messenger Venus - Venus Express Moon - Lunar Reconnaissance Orbiter, ~~GRAIL~~ Mars - Mars Odyssey, Mars Express, Mars Reconnaissance Orbiter Vesta - Dawn (later this year will head to Ceres) Jupiter - Juno (on the way - 2016) Saturn - Cassini Pluto - New Horizons (on the way - 2015) - Flyby only *This list may be a bit outdated.
Since the yeast that makes bread rise works the same way as yeast for making beer or wine, does that mean there is alcohol in bread? If so, how much? How much would I have to eat to get drunk?
Answering as an avid bread-baker and not a food scientist, the internal temperature of cooked bread is around 190F, whereas ethanol's boiling point is 173F. Ethanol is certainly produced by yeast during the bread fermentation, but what's there is likely almost entirely cooked off during baking. I've kept a sourdough starter culture in my fridge for years (basically live yeast, lactobacillus, flour, and water). After it hasn't been used/mixed for a while, a significant layer of liquid forms on the top of the slurry which has a very distinct smell of alcohol.
How come when you look at something very bright like the sun, how come you can still see it briefly when you close your eyes?
Your eyes have little microstructures in them, sensitive to light, that transmit optical information to your brain. If you look at a very bright object, these microstructures will attempt to 'dim' the aggravating wavelength. This effect will persist for a while after looking away. This is also why looking at hot pink images will result in seeing blue when you look away; the 'pink' wavelengths are subtracted from everything else you are looking at.
Is whale sperm any bigger than human sperm?
[In male humans, sperm cells consists of a head 5 microns by 3 microns and a tail 41 microns long.](_URL_0_) Whale sperm vary in length according to species: [73.8 micron for a porpoise, 40.6 for a sperm whale, 52.5 for a humpback whale.](_URL_2_) I couldn't find it for the blue whale though, sadly. I will assume the blue whale's sperm is in the same ballpark as other whales, making it close to human sperm in size. Sperm size doesn't really correlate to organism size; the longest sperm on Earth are from fruit flies; [Drosophila bifurca has sperm 58 mm (2.3 in) long.] (_URL_1_) **Edit:** Links added.
Why is Cloud Seeding Not a Viable Option in Battling the Current Drought?
Just say you don't know... Cloud seeding presumes there is moisture in the atmosphere to "seed" and bring down as rain in the first place. In the case of most droughts that is not the case in the geography needing the rain in the first place - basically you can't squeeze blood from a stone or a turnip. What would be needed is a bit more profound and problematic: you'd need to change global air circulation patterns such that you'd get moist air into the area with the drought. However the very nature of Navier-Stokes Equations which govern fluid dynamics (which is what air circulation is), are notoriously prone to chaotic uncertainty - it's why you can't predict the weather much more the 3-6 days into the future as an inherent limitation of nature, the models and the mathematics. But controlling global circulation patterns would require exactly that, you have an insurmountable barrier to success.
Why is cloud seeding not used more often worldwide to curtail drought?
At best cloud seeding can increase the amount of rain from a rain event. It can not create clouds and rain out of air that couldnt support them to begin with. The current droughts are caused by large scale atmospheric pressure systems diverting moisture and suppressing storm development. If the storms aren't already going to form you won't be creating them by seeding. Overall it doesn't really work that great and costs a bunch of money. _URL_0_
Why do flashing colors/lights induce seizures in some people but not others?
Your question relates to a specific type of epilepsy, generally designated as photosensitive epilepsy. The answer for such question is very much the same as the more general "Why do epileptic brains have seizures?". There is not one single answer for such question, several conditions may lead to recurrent seizures (by definition epilepsy). But the general concept is that an epileptic brain is more easily disturbed from a stable regime, where the brain activity is very much controlled, to an unstable regime where excitation and inhibition are not properly balanced and uncontrolled activity emerges and propagates to different areas of the brain. Now all brains in principle can have seizures given a strong enough insult, what differentiates "epileptic brains" is that they can transit to a seizure state, either spontaneously or with common stimuli, while a healthy brain doesn't.
Could Jupiter have been a star?
Sure, if you multiplied its mass by 80 or so. Like stars in general, its composition is approximately that of the universe in general--mostly hydrogen with some helium and then some trace elements. But in order to get enough pressure and temperature going in the core for sustained proton-proton chain fusion, you'd need to pile on a lot more mass. Theoretical models put that minimum mass in the range of 70-80 times the mass of Jupiter. And even then, it would not be a very luminous star, only putting out around a thousandth as much power as the Sun.
Is there a size difference between the "observable" universe and the Universe?
There is a difference. The observable universe is explanatory - it is what we see when we look. The non-observable universe is the universe in its entirety. That means everything in the universe, even what we can't detect. It's not certain, but the universe probably extends much farther than the observable one, and some things in our observable universe will soon leave our observable universe (because of expansion). How much is potentially there? That's very debatable. Some say it's infinite, some say it's finite, and a few say it's smaller than the observable universe! [This wikipedia article](_URL_0_) might interest you if you want to find out more.
If the "fuel adds weight which needs more fuel" problem means rockets already require 20 times as much weight in fuel as the payload just to get into low earth orbit, would a slightly larger planet mean civilizations can't get to space? What size planet is the limit?
Back of the envelope calculation: if you assume that a given planet has roughly the same density as Earth, then the amount of fuel required is about m e^kr, where m is the mass of whatever you're trying to launch and r is the radius of the planet. (k is a constant sqrt( 8 G pi/3v^2 ), where G is the universal gravitational constant and v is the exhaust velocity of your rocket engine). So it never becomes *impossible*, but the amount of fuel required increases exponentially as the planet increases in size. It would become impractical quite quickly.
What is happening chemically when bread goes stale?
They're attempting to reach an equilibrium with the water content of their environment. Bread, despite being dry to the touch, actually has a fairly high percentage of water bound up in its structure. Chips and crackers, on the other hand, have most of their water driven off during the cooking process. When presented with a typically humid environment, water will (slowly) migrate out of the bread and into the air; conversely, water will be absorbed by the chips and crackers from the air.
Why is it that the sun emits more cyan coloured wavelengths, yet we see it as yellow?
> Why is it then that the sun appears yellow to us? It cannot be due to the atmosphere, because even in space it looks the same. Actually it is mainly due to the atmosphere. [Rayleigh scattering](_URL_0_) preferentially scatters bluer light, so bluer wavelengths get taken out of the light you see from the Sun and get scattered into the atmosphere instead.
Why does the sun appear yellow when its peak wavelengths are blue-green?
Blue light gets diffracted easily, whilst red light doesn't diffract a lot. When the blue wavelengths hit our atmosphere, they spread out creating what we know as the sky. Red light keeps a more direct route, which appears as the sun. The same phenomenon can be used to explain sunrise and sunset skies.
Whenever I clean my ears with Q-tips, I find a certain part of my ear that, when touched, produces a very strong gag reflex. Why does this happen?
The pharyngeal reflex, or [gag reflex](_URL_0_), is controlled by glossopharyngeal nerve to the solitary nucleus in the brain. This nerve receives general sensory fibers from the tonsils, the pharynx, the middle ear and the posterior 1/3 of the tongue. By activating the sensory fibers originating from the middle ear, you will, essentially, be mimicking the effect of those fibers in the pharynx, causing you to gag. Source: Dental Student!
Why is all known life carbon based? Is there any evidence for a life form based on any other element?
> To add to that, is the bacteria found on Mars carbon based? There are no bacteria that have been found on Mars.
Why does burning wood crackle?
Wood that has moisture and is heated will produce a mixture of gasses. These gasses build up pressure from inside the wood until it fails from the stress. The wood cracks releasing the gas producing the sound you hear around campfires.
Why does water that has been sitting for a while taste different than water directly from the tap?
Very little Dissolved Oxygen. Bubbles observed on the walls of the glass of water, is evidence of the out gassing process. Higher Water temperatures hold less Oxygen and also makes the water seem to taste not as "fresh".
Why doesn't carbon have a melting point?
It does have a melting point, just [not at atmospheric pressure](_URL_0_). The process of going directly from a solid to a vapor is called sublimation and it's not that uncommon. Another example of sublimation is the "fog" you see coming off of dry ice.
Is it possible to Melt carbon, and if how? and if not why ?
Yup, it is. Here's a review about the topic: _URL_0_ You need very hot conditions in the 4,000-5,000 Kelvin range and atmospheric pressures approaching *at least* 100 times atmospheric pressure.
Is crude oil considered to be organic?
It's definitely made of organic compounds, mostly. So yes, it would be considered organic. Note, though, that the term "organic" in chemistry means "containing carbon", not whatever it is supposed to mean in the supermarket. There, I would just ignore it with the same reasons as "gene-free popcorn".
Why is it that pi has an infinite number of decimal places? And why does Euler's number also do this?
Let's take a number that has a finite number of decimal digits. For example, 1.234567891011. You can see that it's equal to 1234567891011 / 1000000000000. In other words, it's a rational number (i.e. a fraction). So, the numbers with a finite number of decimal digits are a subset of the rationals. Thing is, irrational numbers (i.e. the numbers that are not fractions) are infinitely more numerous than the rationals. That means, if you take a number "at random," without constructing it so that it's a rational, it will (very probably) be an irrational number. Both e and π have no reason to be rational (because they aren't constructed to be), so, it shouldn't be surprising that they aren't.
Has the invention of light bulbs and electricity affected the evolution of moths?
The favoured hypothesis is that moths are not directly attracted to light; they are confused by light. Night flying insects use the moon to maintain a flight path. In order to fly in a straight line a moth just has to keep a light source (i.e. the moon) at a certain angle. This does not work when the light source is a nearby artificial light because it's relative position changes as the moth flies. When they try to fly straight by keeping the relative position of the light the same their actual course spirals in (or out) towards the light. Unfortunately I don't know of any studies that have attempted to detect a response to selection caused by artificial lights.
What makes static electricity have that distinctive smell?
You're probably smelling the production of ozone by the sparks. Interestingly, the name "ozone" comes from the Greek for "smell". From the [Wikipedia on ozone](_URL_0_): > Ozone may be formed from O2 by electrical discharges and by action of high energy electromagnetic radiation. Unsuppressed arcing breaks down the chemical bonds of the atmospheric oxygen surrounding the contacts (O2 → 2 O). Free radicals of oxygen in and around the arc recombine to create ozone (O3)
Where do dead bacteria go?
Sort answer : yes you eat the dead microorganisms. They are digested and passed just as other organic matter. Longer answer : furthermore, you actually eat live microorganisms all the time. Bacteria are everywhere! Additionally Your gut is actually full of a microbiome containing one of the densest concentrations of bacteria in the planet. In your colon. Most of your poop is live bacteria by mass. There are also many viruses and fungi. There are even virises that only infect the bacteria. (bacteriophages). This microbiome helps you digest things and even provides you with specific vitamins. These microorganisms are replicating and dying inside you at every moment. It is estimated that there are over ten times the amount of bacterial cells inside and on your body than human cells in your body. That's right, you're more bacteria than human by an order of magnitude! This is possible because bacterial cells are considerably smaller than human cells. Feel free to ask any more questions PhD in pathobiology
Waking up from a long Coma : Do people recall and feel as if it was just recent, or do the memories 'age'?
It's difficult to tell. Despite what we see in TV and movies, people who recover consciousness after a long coma do not suddenly come back to normal as if from sleep. There are always significant, long-lasting deficits. Memories from before the coma tend not to be clear and sharp, but it would be hard to distinguish what is affected by the injury compared to what is simply blurred by time.
Why does the gravity of the Sun pull smaller bodies such as asteroids towards it, but not satellites like the Moon?
The Sun *does* exert a force on the Moon, though, and a rather significant one at that. Consider the orbit that the Moon traces out over one Earth solar year. You'll notice that it describes a mostly-circular path around the Sun, following the Earth's orbit. If you were to "zoom in" to the orbital path, you'd see the comparatively small disturbances in the overall orbit made by the Earth, which would cause the Moon's orbital path to trace out a curlicue shape.
Why does my throat hurt when I scream or yell for long periods of time?
This is called vocal fatigue, and I found [this review](_URL_0_) that goes over some of the causes if you can get access to it. Essentially, when you make a sound with your voice, you contract muscles in your larynx to bring the [vocal folds](_URL_2_) together. The vibration of the vocal folds creates sound as air passes through them. If you scream or yell for a long period of time, the muscles in your larynx become fatigued, just like any other muscle does when overused. This fatigue can result in discomfort or pain. There are other things that can happen with vocal fatigue, especially if it happens over and over again, as it often does with singers. There can be growths or [nodules] (_URL_1_) that form on the vocal folds, which prevents the folds from closing properly. And in extreme cases, a blood vessel in the folds can burst (called a vocal hemorrhage), which as you might expect, can be very damaging and requires immediate treatment.
[astronomy] are there planets around the stars we see with the naked eye?
Simple answer to your question: Yes. [Here's a small list](_URL_0_) (as of 2010...PDF WARNING). Generally, anything brighter than magnitude 4 is viewable with the naked eye in urban/suburban settings; dimmer, magnitude 7 objects *may* be visible in rural/very remote areas. Edit: Depending on one's criteria, the probability of a star having a planetary system appears reasonably high (_URL_1_), though better imaging and detection technologies will refine these estimates.
If two identical twin boys marry two identical twin girls, will each couple's respective children be identical?
No more so than any two children of the same parents would be identical. You get half of your genetic material from your mother, and half from your father. But you only get half of each of their DNA in the process.
Why do some things melt and some things burn?
There is an important distinction between burning and melting. Melting is a **physical** phase change from a solid to a liquid while burning or combustion is a **chemical** change. Melting is the resulting phase change brought about by increase in temperature whereas in combustion, the heat is a product of the reaction and not the cause for the change. Wood is a very complex substance comprised of different compounds like water, and many different organic substances (composed primarily of carbon, hydrogen and oxygen). Many individual components in wood have their own melting point. Others like cellulose (the primary wood fiber) decompose or break down into simpler compounds before they melt. Since wood is such a complicated composite substance comprised of dissimilar compounds with vastly different properties, it is impossible to simply heat a piece of solid wood and get liquid wood. EDIT: Grammar
Why do bees die when I spray them with Windex?
Ammonia is toxic, not sure about toxins in febreeze but you maybe essentially drowning them by spraying them, and hair spray has lots of hazardous chemicals.
We can see galaxies created shortly after the big bang because their light is only just reaching us now. But something has always bothered me about this.
I'll just add that you have to take into account the expansion of the universe, which is not limited by the speed of light. There can in fact be galaxies that are "receding" from us at faster than light speed.
Why are diet sodas (i.e. Diet Coke/Pepsi) not sticky when spilled on a surface like their non-diet counterparts if both use sweeteners?
Natural sugars (sucrose, glucose, or fructose) in non-diet sodas have a lot of functional groups that form a special type of intermolecular bond (called a [hydrogen bond](_URL_1_)) with water molecules. It is these interactions that creates sticky solutions of water and simple sugars. Diet sodas taste sweet because [sugar substitutes](_URL_0_) are added instead of the natural sugars mentioned earlier. These solutions are less sticky for two reasons: (a) most of substitutes taste much sweeter than sugar, so much less needs to be added to get to the same level of sweetness, and (b) they often form less hydrogen bonds with water as do sucrose, glucose, or fructose.
Does burning a candle 'clean' the air?
Scented candles can mask the bad odor, they don't do any cleaning what so ever. Infact, burning wax releases soot which is not good for your respiratory system.
Since stars only sparkle due to atmospheric conditions, at what atmospheric density do they lose their sparkle?
Its usually not a specific density (though there may be one beyond which they always sparkle) but how stable the atmosphere is. Even at sea level on very still clear nights the stars don't seem to sparkle. Other times it may be clear but the seeing is bad, stars seem to dance around like at the bottom of a pool. The Seeing has to do with turbulence and temperature changes in the atmosphere between the observer and the star. If there is very little temperature change and its stable Seeing is good, the stars don't sparkle. [Here](_URL_0_) is a more detailed discussion.
Could Stars have rings too?
No. The reason rings look so brilliant is that they're made of ice and so reflect light really well. To look like Saturn's rings, they would have to be very close to the star (about a few stellar radii away). This means that they would be very hot and the ice would melt.
Are any stars known to have rings?
Yes. In fact, we have one. It sits right between the gas giants and the terrestrial planets, and we call it the asteroid belt. If you are looking for something closer to Saturn's rings, where the rings are closer to the surface of the sun, then that is beyond my expertise.
Assuming the theory that comets delivered water to Earth is true, why aren't Mars, Venus, and Mercury also full of water?
Mars has great reserves of water; it's just almost all frozen underground. If I remember correctly, there's enough to cover the majority of the planet to several meters depth if it were all melted. Mercury, also, has some reserves of water in permanently shaded craters at it's poles. This is incredibly surprising because the surface reaches hundreds of degrees C during the day, and it was not expected that water could survive near such an environment for very long. Venus lacks water because, due to the tremendous heat of it's atmosphere, water dissociates into Oxygen and Hydrogen, and the Hydrogen escapes into space. There are no cold craters for the water to hide in; the planet is a single burning hellhole. This process takes millions of years, but the planet is billions of years old. Earth has an atmosphere that can sustain liquid and solid water, and doesn't get hot enough for the water to dissociate at too high a rate and escape. It's a very fine balance.
If our water mostly came from comets and such, wouldn't water be fairly common in other planets?
It appears to be. Almost all objects in our solar system have some, and many outer solar system bodies have a lot of water ice in their bulk. Water is the most common compound in the Universe after all, being made from the 1st and 3rd most abundant elements. What's uncommon is liquid water on the surface, since that requires a terrestrial planet with sufficient atmospheric pressure and suitable surface temperature. In our solar system that's only found on Earth. Mars comes close, it can get warm enough and is thought to have ephemeral water flows, but the air pressure isn't quite high enough so any water on the surface of Mars will boil or freeze. Venus and Titan have enough air pressure but are way too hot and way too cold respectively.
What was the reason that natives died from sickness that the colonist carried like the flu but not the other way around?
Maybe one of the reasons is also that when Europeans contracted exotic diseases that their body couldn't fight, they would die/recover before coming back to Europe where it could have caused an epidemic we would have remembered, as opposed to how Europeans could stay healthy while carrying their domestic diseases over to their colonies? I mean, outside of the good answers that have already been given.
Considering that dreams typically seem like they're actually happening, what keeps us from constantly talking and moving while we dream?
Answered this in a similar question a while back, but, dreams tend to occur during REM sleep. Your muscles are actually paralyzed in REM sleep, and not in delta wave/slow wave sleep (when you sleepwalk). There is a condition called REM sleep disorder in which that paralysis fails and you end up moving/striking out during REM sleep, but, unlike sleepwalking, REM sleep disorder is characteristic of much more severe neurologic conditions (Parkinsons is one of them).
[Neuroscience] How does science explain the fact that even those who were born blind can see in their dreams, as naturally as a normal person?
The answer appears to be no, congenitally blind individuals do not have visual experiences in their dreams ([Kerr and Domhoff, 2004](_URL_0_)). They cite other work that argues that mental imagery is necessary for dreaming and that this only develops between the ages of 4-7. So even for those who are blinded at an early age, but had some visual experience, might still not see anything in their dreams. Edit: Perhaps someone else can pitch in on this, but I wonder how this jives with mental rotation work from infants, namely that infants as young as three months old can (somewhat) perform mental rotation tasks ([Moore and Johnson, 2011](_URL_1_)).
Why are download and upload speeds in average internet connections so radically different?
Simply because most ISPs limit your upload speed to a fraction of your download speed as a cost-saving measure. The idea from the ISPs side is that most of what users want to do is download (streaming music, video, visit web pages, etc.), so they go cheap and limit your upload speed so they don't need as much infrastructure to handle upload traffic. It also discourages people from running their own servers. That's not a technical limitation. Find an ISP that provides symmetric upload and download speeds, and you won't see much of a difference.
What is the efficiency of riding a bicycle versus running / walking?
> On firm, flat, ground, a 70 kg person requires about 30 watts to walk at 5 km/h. That same person on a bicycle, on the same ground, with the same power output, can average 15 km/h, so energy expenditure in terms of kcal/(kg·km) is roughly one-third as much. Generally used figures are > 1.62 kJ/(km∙kg) or 0.28 kcal/(mi∙lb) for cycling, > 3.78 kJ/(km∙kg) or 0.653 kcal/(mi∙lb) for walking/running, > 16.96 kJ/(km∙kg) or 2.93 kcal/(mi∙lb) for swimming. And there's more, all on [Wikipedia]. [Wikipedia]: _URL_0_
Why is driving a bicycle more efficient and less tiring than walking?
In addition to the physics mentioned, keep in mind that when you walk you are supporting your entire body weight and when biking you are supported with a seat and your legs are now only needed to push on the peddles. Generally much less force is needed. This is why when someone is OW or OB it is easier to do maxVO2 tests on a stationary bike than a treadmill. They don't have to support as much body weight while on the bike and you may be able to get a more accurate value as on a treadmill they may fatigue far before they reach a plateau in their VO2.
Why do my mashed potatoes take so much longer to microwave than my turkey or green bean casserole?
they have different density (amount of mash potatoes per volume is higher than beans?) and different heat capacities (how much energy is needed to raise the temperature by a given amount). The most important factor is most likely water content, microwave ovens work by heating up water within the food, the more water the quicker the food heats up.
What's the chemical group that is responsible for the distinct smell of freshly cut grass?
The chemicals are Green Leaf Volatiles, which are a variety of organic compounds. _URL_0_ Apparently, the smell is a defense mechanism for plants. While being eaten, they give off the smell which attracts predators as an indirect defense. _URL_1_ Neat!
Do astronauts on the space station get sick?
I just did a research paper on this subject! While it doesn't happen too often, some dormant pathogens can become active after the body is exposed to the stresses of being in space. Edit: Space is a very stressful place and it takes a toll on our immune system, this allows for dormant pathogens as well as normal bacteria to "take hold" and cause illness. Though, because of the regulations in place there's never really been a medical issue from it. So, in short yes it is possible and happens.
How do artificial sweeteners like Splenda create the sensation of a sweet flavor while not containing any calories?
They're compounds that bind to the receptor proteins on the tongue but are either so sweet that their caloric contributions are negligible or they are incapable of being broken down by enzymes in the body to produce energy. Aspartame is an example of the former, it has roughly four Calories per gram, but is 200 times sweeter than sucrose. Sucralose is an example of the later, at the same time being up to1,000 times sweeter than sucrose.
I remember reading that a storage device full of data has slightly more mass than one that is empty, is this correct?
[Here](_URL_0_) is a video explaining the mass of data. Take home is that the entire internet weights about as much as a strawberry...
Why does the moon vary in size from day to night, or even from night to night?
If you'll permit a Wikipedia article on the subject: [Moon Illusion](_URL_0_). Basically, objects near the horizon appear bigger, but in terms of angular size the object experiences no actual change throughout the objects path through the sky; it's all a perceptual illusion. In terms of your experiences, I would argue that you have a greater tendency to notice the moon at night than when it's close to the horizon early in the day. The moon will naturally not be close to the horizon during the day (*before* sunset) unless it is close to a full moon (waxing), and you might have a tendency to notice it higher in the sky when there's a larger contrast between the white of the moon and the blue of the sky.
What are the differences between Human and Animal antibiotics?
Chemically the active ingredients are exactly the same. The same drug in the same dosage *should* be compatible with multiple species, including humans, in theory. The difference comes in the quality control - it is much stricter for drugs intended for human consumption. I'm not an expert on the exact regulations, but I know that you have a much greater chance of having something you don't want in your body to be inadvertently included in pet medicine than in human medicine. It's similar to the distinction between pet food and people food. The former may be safe, cheap, and marginally edible, but that doesn't mean you really should be eating it.
Are different antibiotics better for specific tasks than others?
There are many many classes and subclasses of bacteria. Some antibiotics are broad-spectrum and target most anything, others are narrow spectrum and target only specific families. Different antibiotics have different potencies and side-effect risks. Sometimes the choice of antibiotics is somewhat arbitrary, other times a very specific drug is warranted. It all depends on the situation. Questions like this is what pharmacists are for (not to mention the prescribing doctor)
If a child is raised in a household that exposes them to two different languages equally, does this harm learning when starting school?
This would likely enhance the ability of the child to learn.
Did human beings evolve in Africa or in many places simultaneously?
The current prevailing theory is the out of africa hypothesis, that is a small number of humans i think the number may be as low as 600 breeding pairs evolved in africa and then spread out across the world. This is supported by the limited amount of genetic variability found in modern humans. If ama had evolved across europe and africa we would expect to see more variation. However, it appears that ama's bread with Neanderthal in europe as a consequence africans do not have Neanderthal dna whereas people from the rest of the earth have a very small amounts.
At what point can we call different breeds of dogs separate species?
All domestic dog varieties are still considered part of the species Canis lupus familiaris. Through artificial insemination, theoretically it should be possible to produce a chihuahua/great dane mix (although virtually impossible otherwise). They are part of the same species because they have not evolved to the point where they are reproductively isolated. The reason we see such a vast array of dog breeds is because over many years dog breeders have selected for certain traits and characteristics and bred dogs accordingly. The accumulation of these traits over time will lead to a new breed, but it would take a much greater period of time for those breeds to become separate species. Genetically, they are still almost identical even though they may be morphologically very different. Hope that helps!
Can sudden bursts of loud noise (specifically through headphones) cause significant hearing loss?
The external noises you hear come from sounds waves entering the ear canal. These waves are physically transmitted to the inner ear, where they bend tiny, hair-like structures called cilia on "hair cells." As the cilia bend, they transmit signals to the brain, and this information is processed, giving you the experience of sound. Extremely loud noises, including the ones you described, can damage the hair cells and prevent the cilia from bending properly. This can eventually lead to irreversible damage and subsequent hearing loss over time, usually with the person losing the ability to hear high-frequency sound first. The exact mechanism of how loud noise can lead to hair cell death and eventual noise induced hearing loss (NIHL) is still under heavy investigation in the field. Some recent theories involve the production of reactive oxygen species (ROS), but the risk is certainly a mix of both environmental exposure and genetic factors.
How do hackers exploit 'weak' passwords for online accounts?
Enforcing a password length and locking accounts are separate countermeasures to prevent brute force attacks. In older bits of code, for instance, you often get functions that leak length information about the password during the database comparison. All brute force countermeasures are essentially about increasing the attack time to infeasible levels.
How is it possible to brute force a password when most computers/websites lock a user out after a low number of incorrect tries?
To check if someone typed in the correct password, the computer (server or local computer) calculates a hash of the typed in password and compares it to the stored hash of the correct password. If both are identical, the user knows the correct password. Hashing a text produces another text, the hash. This hash can't be reversely computed into the original password, therefor you can't tell the password by looking at the hash. For brute forcing, you need the hash. For a local computer, you can just read it from the drive (if the drive isn't encrypted). For a web service, you may get the hashes from a security leak of the service, which are often uploaded after a breach. When you have the hash of a user, you can begin to try out passwords, hashing them and then see if the resulting hash is the same as the one you are brute forcing. If they match, the correct password is the one that produced the identical result. Because everything is local now, you can try as often and fast as you like.
Why can looking at a solar eclipse blind you in seconds versus looking at the sun normally?
The "blind you in seconds" part is an exaggeration, but there is an explanation for why looking at a solar eclipse can be more harmful than looking at the sun normally. When you normally step outside into the sunlight your retinas contract to limit the amount of light coming through. The sun puts out a lot of power you know, more than our eyes can handle directly, though this contraction of the retinas helps protect you. During an eclipse though, everything gets darker and your retinas dilate somewhat to allow more light in. When the sun pops back into view your dilated retinas allow much more of that harmful light in than usual, possibly causing damage. EDIT: the contraction/dilation are your irises, not retinas. (thanks cowboysauce)
How strong/durable would a sheet of diamond be?
Diamond is the hardest naturally occuring substance, which means it's scratch resistant. But it doesn't mean it won't break. That's more of a toughness thing. Toughness measures how much energy it takes to break it. Diamond has a toughness of about 2.0 MPa m^(1/2). Glass is about 0.7 to 0.8 MPa m^(1/2). So diamonds would be about 2.6 times tougher. Imagine throwing a rock to break a window. You'd need to throw a rock 2.6 times heavier the same speed or the same rock 1.4 times faster to break a diamond window than a glass window. In case you're wondering, strength refers to how much force it takes to break something. If it flexes more, a material that's just as strong can be tougher. I can't find anything saying how strong diamond is though.
Generally speaking, are audiobooks as good for you as regular books? Have there been any studies comparing the two?
The main advantage of reading text instead of listening to audio is the ability to go back and reread passages. This is one criticism of audio literature, but could also apply to reading for the purpose of learning new information. For example, when a new term is defined or concept is introduced, you may reread that text several times immediately or return to reread it later for clarification.
Is the universe expanding faster and faster at the same speed everywhere or is there areas where it's expanding at a different speed?
When you look at scales above the size of superclusters, space seems to expand at the same rate in any point of the observable Universe, as far as we can tell. This means more precisely that uniform expansion is a good fit for the available data and introducing inhomogeneities does not actually help reproducing that data better, so for the sake of simplicity (or Occam's razor if you want) we state the Universe on cosmological scales is homogeneous, a fact which is part of the Copernican principle. However, there is no reason for this to hold to arbitrarily large scales. Perhaps the Universe along with its expansion rate *is* inhomogeneous over lengths much larger than the radius of the observable Universe. There is really no way for us to know experimentally, but it is also not an idea to be discarded - some theoretical models of cosmology generate this type of inhomogeneous "bubbling" universes, an interesting example being eternal inflation.
Is climate change related to heat capacity of combustion product gases relative to the starting gases?
You have misunderstood the mechanism from the beginning. The basic concept of greenhouse gases is that the light from the sun (which peaks in the visible spectrum) can mostly pass through. This light hits the Earth and is absorbed, warming the Earth. But the thermal radiation emitted by the Earth is mostly in the infrared. The greenhouse gas absorbs strongly in the infrared, so it reduces the amount of radiation that escapes the atmosphere. This causes the Earth to warm. The heat capacity is not related to this phenomenon. e: [That said, specific heats *are* related to the absorption/emission spectra, but in a complicated way.](_URL_0_)
Why don't the gases of Jupiter mix?
They do. The color bands are just slight chemical and temperature variations of the same stuff. But here's the cool part - we see them as bands because those are convection currents. There's so much heat in the middle of Jupiter, and Jupiter is rotating so fast, that the various convection areas get formed into bands by 370 mph winds. The light colored bands are actually at a higher altitude than the dark colored bands, too. In short, Jupiter is one weird place.
How “flat” is the universe?
The Universe is not flat in that sense. It is, as far as we can tell, the same in every direction and every place (once you average out over large scales). That being said, it can have a geometry. For example, going one dimension lower, you can imagine that it might be flat like a piece of paper, or curved like the surface of a sphere. Both those surfaces have the property of being the same everywhere and in every direction, but they have different geometries. For example, the angles of a triangle on the sphere don't add up to 180 degrees. Also, of course, if you go all the way around the sphere, you come back around the other side. The Universe can also have 3D versions of those geometries. (It could also be "open" like a saddle shape.) All the evidence to date is consistent with the Universe having a flat geometry. By the way, flatness of things like the solar system is due to their rotation. There is also no significant evidence that the Universe is rotating.
Why is it that when you stare into a light bulb you can shut your eyes and still see a glowing image of the light.
The retinal photoreceptors are stimulated and likely continue to be stimulated after a bright light exposure. As long as they continue to be stimulated, a signal is sent to the brain that an light persists (positive afterimage). Eventually, it will fade as the stimulation decreases and may persist as a negative afterimage. Source: I am an ophthalmologist and have access to wikipedia _URL_0_
Are electrons, protons, and neutrons actually spherical?
Recent experiments have demonstrated that electrons are indeed "spherical" (i.e., there are no signs of there being an electric dipole moment in the electron). Or at least they're spherical to within 1*10^-29 cm. Scientists have observed a single electron in a Penning trap showing that the upper limit for the electron's "radius" is 10^-20 cm. So that means electrons are at least 99.999999999% spherical! Read the recent experiment: _URL_1_
Are electrons, protons and neutrons actually spherical?
Well....in the case of the electron, the question is already a little dicey, as thinking of an electron like a tiny billiard ball really isn't quite right. When you look at the wave particle duality aspect, ascribing a specific 'shape' to an electron doesn't make as much sense as you might think. Then if you go deeper into quantum field theory, an electron is a localised probability density in a field that expands throughout all spacetime, and its movement is a perturbation of that field, so that gets even stranger. However, that aside, they certainly appear as if they are spherical by the best observations to date. There is some proof that they appear spherical, and you can read about it here. [Electrons appear to be spherical.](_URL_0_)
Why is helium the second most abundant element in the universe?
Helium is a product of hydrogen fusion, and hydrogen is the [most common element](_URL_1_) (note log scale). If you want to play a bit with stellar fusion cycles, have a go with this game: _URL_0_
If Helium is the second most abundant element in the universe then why is there a shortage on earth?
crnaruka's answer is good, but is it possible or economical to harvest it from space? There's certainly a lot of it out there. Jupiter's atmosphere is 25% helium. If you ever need to inflate a lot of party balloons that would be the place to go. There are proposals to [mine the moon for helium](_URL_5_), but for fusion reactors and not for balloons. Helium is blown away from the sun in the solar wind and becomes embedded in moon rocks, and the isotope helium-3 is a possible fusion reactor fuel. It requires higher temperatures than other candidates and mining the moon is not going to be easy, so this is deep in the realm of science fiction for now.
Does the number of other groceries in my fridge affect how long it will take for a room temperature pitcher of water to cool down?
The less space for air in your fridge means less heat transfer every time you open the fridge door. This means that a full fridge (containing already-chilled groceries) has a head start when you first place the pitcher in, while an empty fridge needs to spend extra energy cooling the air down again. That said, assuming the starting conditions were somehow identical (as in, you teleported the pitcher without opening the door), then the extra air would aid in cooling, as it can circulate the heat away from the pitcher better.
Can you freeze gasoline and if so for how long and can it still be used once thawed?
Gasoline can "freeze" as in get hard when cooled enough, but it doesn't freeze as water does, by forming a crystalline structure. Gasoline is actually a mixture of substances with different properties, and freezing points. Gasoline also has a [flash point](_URL_0_) of -45F, which is the temperature above which it will burn. So it will burn even very cold. [Here](_URL_1_) is a video of somebody freezing gasoline and lighting it on fire.
Far away stars delivering photons directly to my pupil?
You are correct. If you can see something, it is because photons traveled from that thing directly to your eye. Unless there was some reflection involved, or a refraction. Then its path was slightly indirect.
Why do I tear every time I yawn?
The reason that we always tear up is because when we yawn, the muscles in our face are so contorted that pressure is put on the lacrimal glad in the face ([diagram](_URL_0_)). The pressure on the facial muscles then pushes out tears from the caruncula. Pretty simple process, actually!
Why do our eyes tear up when we yawn?
[Diagram.](_URL_0_) Normally, tears go from the lacrimal gland, across the eye, into the lacrimal sac, and finally drain away through the lacrimal duct. Note that the lacrimal sac acts as a reservoir for the tears to drain from. When we yawn, our face muscles apply pressure to the lacrimal sac, which causes an excessive amount of tears to be pushed back onto the eye. Additionally, the lacrimal duct is also closed off by the pressure so the tears can't drain.
Apparently there is an upper limit of the mass of a photon. Would a massive photon be bad news for physics?
This paper discusses the ramifications, and is fairly readable: _URL_0_ Mainly, a photon having the maximum possible mass that could decay into the lightest neutrino, given how thermal the cosmic microwave background is, would have a lifetime of about three years, which is extended to much longer than the current age of the universe because of relativistic effects. The paper also talks about the implications that it would have, and they aren't disastrous. The biggest thing is that "the speed that light travels" and "the speed associated with Lorentz transformations" are no longer the same thing.
Why are penises darker than the rest of the body?
[Pheomelanin](_URL_0_) is a pigment that produces a pink/red hue to the skin, and is found in higher quantities in the tissue of the penis than in other skin areas.
Why does rubbing the bottom of a can of soda that I just dropped prevent it from 'exploding'?
Are you sure this is actually the case? I am willing to guess that if you actually ran a small experiment (buy a 12 pack of sodas, drop them all from the same height, rub the bottom of 6, and don't rub the bottom of the other 6), this wouldn't have much effect. But I could be wrong.
When salt (NaCl) is dissolved in water, why doesn't the sodium react violently with the water?
Yes, it does have to do with the fact that they're ions. Non-ionized sodium really wants to donate its valence electron to anything that will accept it. Also, non-ionized chlorine wants to steal an electron from anything it can in order to fill its valence shell. This is why sodium reacts violently with water, and why chlorine is so dangerous. When they're ionized, the danger is gone.
Why when salt (NaCl) is dissolved in water it doesn't act like Sodium and Chlorine?
Chemical reactions are driven by exchange of electrons. When dissolved in water, NaCl separates into ions Na^+ and Cl^- , which do not react the same as their atomic counterparts because they have a different number/configuration of electrons.
Helium balloons only deflate and drop because the helium leaks through the latex. Is there any material that is light enough to float without leaking the helium?
Yes, there is - a plastic called mylar. It is often aluminized - coated inside with a thin coating of aluminium. It is the stuff that potato crisps packets are made out of, and it is used for those shiny, metallic balloons. And, unless pierced by rough handling, mylar balloons last practically forever.
Why is fish meat white while meat from mamals is red?
Fish have lower levels of myoglobin which is the protein in muscles responsible for the red colour due to a contained Iron atom. The myoglobin protein is similar to haemoglobin in blood, it carries oxygen. However myoglobin is fixed in the muscle and acts as an oxygen store for when the muscle is under heavy aerobic strain (needs more oxygen). Since fish have their mass supported by the water, they tend to not put such a strain on their muscle aerobically and the myoglobin is needed in lower amounts, although it is more concentrated in the tail due to constant use. Additionally it has nothing to do with egg white protein (ovalbumin).
If we can swap our Sun with a black hole of equal mass planets orbits would remain the same, but how big would this black hole be?
The event horizon of such a black hole will have an area of just about 110 km^(2). If you had a sphere with this surface area that *wasn't* so massive—say a strangely round asteroid—it would be about 6 km across. A lot of sources will quote this as the diameter of a black hole (usually by referencing the "[Schwarzschild radius](_URL_0_)", which is 3 km), because there is a simple formula relating the surface area to the Schwarzschild radius that matches the relationship between area and radius in classical geometry. However, because of the spacetime curvature in the vicinity of something like a black hole, it isn't really possible to interpret a black hole as having a well-defined radius in the sense of "distance from the center", so saying that a black hole is "6 km across" or "has a radius of 3 km" isn't quite right.
Why do radio waves go through objects, but light doesn’t, even though they are both made of photons?
Visible light does go through objects. If it didn't you wouldn't be able to see through your windows. The window on your microwave oven is a good example of something where visible light passes through but radio waves are blocked. Whether or not a given wavelength of light can go through a substance is down to it's properties and how they interact with photons of a given energy.