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ELI5: How come there are some automated body functions that we can "override" and others that we can't?

Answer 1: For stuff like breath and blinking, those are controlled by muscles that receive signals from our brain to contract and relax. This is why we can override those actions.

For heart rate, it is controlled by pacemaker cells that are independent of the brain. Another example is when the doctor taps on your knee and your leg kicks, you can’t stop it. The signal never actually reaches your brain, just to your spinal chord and back.

You can indirectly control your heart rate by influencing it with other factors (movement and breathing). But you cannot only change your bpm through sheer willpower. (source)

Answer 2: The heart and the intestines have a type of muscle called cardiac and smooth muscle respectively. These muscles are not innervated by nerves that can be voluntarily controlled by the brain.Breathing and blinking functions are controlled by skeletal muscles (like your arms or legs) which are innervated by nerves that can be voluntarily controlled.Extra info for the curious: blood vessels also have smooth muscle cells that control the width of your vessels to adjust your blood pressure. These are all innervated by the sympathetic nerves (involuntary control).You have probably heard of the fight or flight response. They use the sympathetic innervations to STOP your gut functions (because they are useless in a life or death situation) and INCREASE heart rate and blood pressure. (source)

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ELI5: Why does the moon look huge in the distance when popping over a mountain but small on a picture or a video?

Answer 1: Not sure why everyone is explaining why the moon looks bigger near the horizon compared to up in the sky when the question is specifically about comparing it to a photo or video.The answer is when you take a photo on your phone, your phone has a wide angle lens which tries to get a wide field of view. I.e it tries to capture the entire scenery in front of you. Distant objects look smaller the wider your lens is.To get around this problem you need to use a telephoto lens. Telephoto (zoom) lenses make distant objects appear bigger because they have a narrower field of view.To make the moon still appear bigger you could include a distant object in the picture like a building or an airplane.The relative size of the distant object to the moon will make the moon look huge. To further ELI5: Hold you hands out wide in front of you. Imagine everything within your arms appearing in a photo. How small would the moon be in that photo?Now bring your arms closer together centered around the moon and imagine this is a new photo. How big would the moon be in this new photo? Bigger right?This is effectively what happens in a wide angle and a telephoto lens photo.Your eyes work a little differently in the sense that they have a wide field of view, but your brain is better at selectively focusing on something in that view - like the moon. (source)

Answer 2:  In addition to the other answers, in photography and film, you can use certain lenses and techniques to make the moon look gigantic, while the camera on phones and a lot of other things generally do the opposite. It might look smaller in the picture than it does irl because the camera being used creates the illusion that it is smaller than it really is. (source)

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ELI5: Why were prehistoric animals so much bigger than their present counterparts?

Answer 1: As a general rule of thumb: animals gradually evolve to larger and larger sizes as the environment permits, until the a change in the environment occurs (drought, food source goes extinct, etc) and then that large animal goes extinct because it has evolved to a size that required all of the existing food and is adapted to a very specific climate and any change in either of those things will lead to extinction.Prehistoric animals had tens of millions of years of gradually reach enormous sizes (the Cretaceous Period lasted about 79 million years) whereas the last ice age ended only about 11,000 years ago. Anytime there is a dramatic environmental change, the massive animals tend to go extinct and the smaller and more environmentally adaptive species tend to survive and be the dominant lifeforms for the next age.When you have a climate that is rich with biomass and it doesn’t change dramatically for tens of millions of years, there is sufficient time and material for massive animals to evolve. (source)

Answer 2:  It was thought to be because of higher oxygen levels and greater land masses. More recently we believe it's because of their physiological features hollow bones supporting more weight and bigger lungs supporting more oxygen growth. These along with the time allowed to evolve are our current theories on why so many prehistoric creatures were large. (source)

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ELI5: Why is hot water more effective than cold when washing your hands, if the water isn't hot enough to kill bacteria?

Answer 1: For the purposes of hygienic cleaning (killing germs, removing dead skin, cleaning a wound), temperature doesn't matter and (in some scenarios eg washing off bodily fluids or with certain soaps.) cold water is actually preferable.For the purposes of cosmetic cleaning (washing off stains, cleaning oily fingers, greasy marks), hot water can help soften long chain hydrocarbons like waxes, grease or oils and can help solubilise inks or other chemicals into the soap or water. TL;DR (Better ELI5) is: If you want to kill germs, temperature doesn't matter. If you want to clean dirty hands, warm water can help.In both cases, washing thoroughly (at least 15 seconds) with soap is the most important thing. (source)

Answer 2:  Soap actually works better at body temperature (which is not hot). When water of any temperature is in contact with your body, it tends to cool or heat to (you guessed it) body temperature. The temperature of the water has been scientifically proven to do almost nothing to the effectiveness of washing - in some soaps, warm water actually causes issues. (source)

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ELI5: Dinosaurs lived in a world that was much warmer, with more oxygen than now, what was weather like? More violent? Hurricanes, tornadoes? Some articles talk about the asteroid impact, but not about what normal life was like for the dinos.

Answer 1:  The level of oxygen wasn't really that much of a factor. Oxygen levels were higher because plants were sucking all of the carbon dioxide out of the air and trapping the carbon into coal and oil at the time while breathing out oxygen and raising the levels up to about 30%. (It's 21% or so now). That much higher level would have made fires way more dangerous in dry areas like grasslands with lots of fuel. Large fires can contribute some to weather, but they usually don't amplify storms in general.The biggest influence was continental structure. We had two different supercontinent-type land formations back then, Pangaea around 300 million years ago broke into two big chunks, Laurasia and Gondwana, during the time of the dinosaurs.Now very generally speaking, the more you pack land into one area and ocean into the other, the greater the general impact on weather... and with supercontinents leaving gigantic stretches of ocean pretty much wide open, you're going to get this to happen. This is because hurricanes feed off of warmer water and shrink when they cross land, and when there's more warm water, there's bigger hurricanes or typhoons (and this is why Pacific storms are often larger than Atlantic ones).Other storms can get amplified too. Nor'easters (the big storms we get here on the NorthEastern coast of North America) build off of differences in air pressure which are caused by differences in heat level. . Larger masses of solar-heated continuous land mean greater regional heating, and that can translate to differences in regional pressure colliding with each other and generating much more powerful localized storms.There's a number of other factors including sea depth (shallower seas warm up more), mountains that deflect currents of air, ocean currents (that help to convey warm and cold weather and equalize temperatures), and distribution of land versus water at the equator where the most solar energy is focused. All of this stuff is why it's hard to talk about specifics back then.But in general, you could expect to get truly massive storms crossing over the coasts of the supercontinents in this altered world. TL;DR: Oxygen, not so much. But the supercontinents back then could really have amplified weather conditions.(source)

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ELI5: If our tears are salty, by don’t hurt, then why the hell does it burn when you get saltwater in your eye?

Answer 1: The salt concentration in sea water is about 3.5%. The concentration that would be isotonic to your body is 0.9%. So sea water is 4x the concentration. (source)

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ELI5: Why is it easier to set a piece of paper on fire by it's corner than on it's center?

Answer 1: Like you're actually 5 > Lighting the edge is easier because it's like lighting both sides at the same time.

Answer 2:  Most likely about surface area, heat distribution, and oxygen content. Fire needs fuel and oxygen to burn, and there's more oxygen on the side (above and below) than the middle (just below). Paper needs to be at 452 degrees farenheit to catch fire. It's easier to catch the corner on fire because you're focusing on a certain point, where in the middle the heat is being distributed. (source)

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ELI5: What happens when a tap is off? Does the water just wait, and how does keeping it there, constantly pressurised, not cause problems?

Answer 1: There isn't a huge amount of pressure there, and it's passive.It's like when you have a water-tank with a tap at the bottom. The water doesn't know a tap is there, until it's opened. (source)

Answer 2:  The amount of pressure in the pipes is not enough to damage iron, copper, PVC, etc pipes. If it were, water would shoot out at extremely high and dangerous speeds when you did open faucets.  And pressure is not cumulative over time, i.e., it does not build up, and the pipes experience the same stress as day one as day 10,000. As long as they are properly installed and maintained, modern plumbing can easily outlast the rest of the building. (source)

Answer 3: To expand on the topic, if the pipes ARE drained, say to repair a leak, when you turn the main back on and open the taps you will get a lot of sputtering as the air is forced out of the pipes and they fill back with water. This would be quite inconvenient if it happened every time you used the tap.Also, in cases like a winter vacation home that's not being used for long periods of time, water MUST be drained from the pipes. When the home is not heated, the pipes can get cold enough for water to freeze. Freezing water expands, bursting the pipes. When it gets warm again, big problems. (source)

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ELI5: How do series like Planet Earth capture footage of things like the inside of ant hills, or sharks feeding off of a dead whale?

Answer 1:  Some of the series at the end do a brief section about how they go about capturing the footage that they showed. They make their own custom rigs with various types of cameras to help them get shots. They leave camera 'traps' in places and hope to get lucky with them. They wander around following research or local guides to help increase their chances of being in the right place at the right time. So a lot of it is somewhat down to luck. They will know from research roughly where to go for certain things, but being able to capture specific things is down to luck on whether they get any usable footage in the days they allocated at a site. Depending on what they are looking to film at any given site, the time they allocate will differ. (source)

Answer 2:  The snow leopard scene in Planet Earth was THREE years of trying to film it. After only getting about an hours worth of filming the animal asleep, and just as they decided to give up, they captured the hunt scene that made in the show. (source)

Answer 3: Videos like these they compiled from thousands of hour of footage over a long time. Planet Earth took 5 years to make.A camera person could be set up in a location recording several days worth of footage of nothing but trees before finally getting the 10 second clip of a moose walking by. Then they'll typically follow the animal several days. There’s not much of a difference in skill/dedication between a scout sniper and a wildlife photographer, other than one shoots with a gun the other shoots with a camera. (source)

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ELI5: Why is there such an emphasis on not consuming plastics and other harmful products rather than on their manufacture?

Answer 1: Companies like Coke switched from reusable glass to plastic because it was cheaper. When plastic waste became a big problem, they paid a lot of money to run ad campaigns that make it sound like it’s our fault for increasing plastic waste and not their fault for producing it all.

When big companies spend lots of money to push an agenda, it usually works.

Just to clarify: The OP didn’t ask “why is there litter” he/she asked “why is the emphasis just on consumers and not producers”. Plastic waste is a complex subject that isn’t easily ELI5. The OP’s actual question however has a pretty clear answer. (source)

Answer 2:  Telling consumers “Stop buying stuff that is bad” works slightly better than telling companies “Stop making stuff that people pay you money for.”

Answer 3: The simple answer: companies don't care, and you can't expect them to do so. Pollution is what we call an externality, something you do that affects other people without you receiving any impact of it. When externalities happen (good or bad), the market does not work well cause you are not considering what your actions cause on other people.In this case, companies trash the environment and don't account for the fact that it harms other people. To fix this, they need to start accounting for it, by getting punished to do so. You can't expect them to turn down a bigger profit for the environment, corporations are rational, they don't have feelings. What we can expect is that if people are willing to punish them by consuming less of their product, they will start controlling themselves. Demand generates supply.This is not really the final solution, it is more like a hotfix. The only real solution is to have an entity that oversees the efficient use of the environment, that punishes or forbids companies from misusing them. This is the role of the state. It is hard to imagine a final solution without a true democracy. Limitations to democracy like lobbies, lack of transparency and misinformed people are the real threat, not the companies. (source)

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ELI5: Why are all economies expected to "grow"? Why is an equilibrium bad?

Answer 1:  Innovation and increases in technological capacity should be cumulatively and continually increasing. So long as this is true, then the economy should naturally grow with it. (source)

Answer 2: The answer is quite simply that the economy doesn't have to grow. But we expect it to because of productivity-increasing technological advancement. When productivity rises, people can make more things with less time and other resources. We get richer.A steady state economy has been heavily analysed and was life for most of human history. It was a steady state because of the lack of technological advances causing higher productivity and higher output.There's also a difference between equilibrium as in no movement and equilibrium in economics: the latter describes forces that return a market to its clearing state.However that is, answers talking about interest rates driving growth are wrong on a number of fronts. First, interest rates are a price. Prices are determined by supply and demand, not vice versa. Second, investment happening does not magically create growth opportunities. The relationship goes the other way: growth potentials happen and people invest in them. Third, a steady state economy (ie where c_t = c_t+1) will have positive interest rates based solely on liquidity preference: that people prefer to have cash now and are willing to exchange a series of payments (ie interest) for it. (source)

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ELI5: When dams are being built, how do they build it with all the water still there?

Answer 1:  They dont. They divert the water then build the dam. Then they divert the water back to where the dam is! (source)

Answer 2: They typically build a "cofferdam" to temporarily divert water around the dam site. Then they build the dam, and once that's built, they remove or destroy the coffer dam.The coffer dam does not need to be particularly big or strong to divert a river. The actual dam needs to be big and strong to hold back the reservoir that's created by the damFun dam fact: Glen Canyon Dam in Utah Arizona is one of the largest in the US, and is under threat by all the sediment that the dam has trapped from moving downstream. The millions of tons of sediment are pushing on the dam and may eventually cause its collapse. So just dredge the sediment right? The problem: Some of it is highly radioactive from all of the uranium mining that took place in Utah and Colorado in the middle of the 20th century. (source)

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ELI5: Why do Marvel movies (and other heavily CGI- and animation-based films) cost so much to produce? Where do the hundreds of millions of dollars go to, exactly?

Answer 1: I work in film and have a VFX degree and here's how it goes:

About half the money, give or take, is for above the line talent. So you have your actors, directors, producers, ect. They get paid in a percentage or in absurdly high amounts for films. These people are also accommodate on set so production has to rent out luxury campers to house them for weeks or months at a time when on location. Then they need to hire drivers and trucks to move those campers. Top tier stars can make demands on top of that. I saw Jim Carrey's camper once and it had an entire astroturf lawn on top of it, with a picnic table, with a vase with flowers on it. Don't ask me why he wanted it, he just did. Those costs are in addition to percentages given to the talent directly, which can be millions each for an A list celebrity. If this is a movie like Infinity War you have multiple guys like RDJ and Cumberbach and like four guy named Chris who could carry a blockbuster on their own and want to be paid like it.

Actors who aren't the main cast still have to show up and get paid. Every random dude you see in the background is an actor who's in it to get paid. If you see a big crowd shot of like 500 people that means that's 500 people who had to show up, go through makeup and costumes, and be accommodated and then be paid.

What you have left over has to pay for production. At minimum it costs like thirty thousand dollars a day just to hire people to actually operate the cameras and set up lights and they usually work 12 hour days and have unions that demand good rates including overtime. This is a very basic cost for a minimum crew for a single day where you get maybe a few minutes of footage done. If you have those big 500 background days you need people to get people to manage those people. If you have complicated shots you need more people for that.

If you're out on location you need to pay the people who own that property. This can cost millions in and of itself if you need time and they know you have money. You also need to pay an entire team of people to show up and get the location ready, which means emptying out whatever furniture is there and replacing it with your own stuff you have to buy. These people are probably also working heavy overtime and have a union demanding pay accordingly. If you decide that isn't worth it then you need to get a studio and build the entire fake set from scratch, or pay a company to recreate it with CG, which isn't cheap either way.

This doesn't count the cost for pre and post production, which is two thirds of the process. You have writers, editors, storyboarders, previz, color grading, foley, and a dozen other departments that have to do work before or after the actual shoot. CG comes here in various phases and obviously isn't cheap. On a Marvel movie if you sit through all of the credits you'll usually see like 8 other companies contracted out to do this and that and if you actually follow through and look up those companies they have big impressive shot breakdowns of what they did and a crew of a hundred plus people who may or may not also be credited.

If you sit through the whole credits of a Marvel movie you probably have thousands of individual names and there are probably three digits worth of people who didn't even make that list. Those guys don't work for free. This shit ain't student film. (source)

Answer 2: CG is very expensive. CG artists are specialists and in high demand. Making a big budget CG blockbuster like an Avengers film employs hundreds of them for years. The personnel costs alone are crazy.

Actually rendering all that CG also eats up a huge amount of time on very valuable, very powerful computers.  It’s also each effects department needs to be on the same page, hundreds of people and dozens of companies/departments are making it but everything needs to look like it was done by the same person with the same eye for lighting and realism, otherwise one shot will come out slightly wrong. That’s not easy. (source) Explain Like I`m Five: good questions, best answers.

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ELI5: What actually happens when we unintentionally start to drift off to sleep but our body suddenly "shocks" us awake?

Answer 1:  Looks like the reaction is not understood, but is probably the activation of the "reflex to stay upright". When your muscles relax when you fall asleep, it may accidentally be interpreted as weightlessness (falling), which may trigger the response. (source)

Answer 2: The different parts of your brains that control what you do talk to each other through waves of electricity when you're awake. When you start to sleep, one big sleep wave goes over your whole brain so the different parts of your brain can't hear each other anymore over this big wave.When you feel like you're falling asleep little by little, almost rhythmically, that is the sleep wave trying over and over again to stop the parts of your brain from talking to each other. When you suddenly wake yourself, that's one or more parts of your brain sending waves out "louder" because they don't know why they can't talk to the other parts anymore. Then the big sleep wave has to try again, and hopefully it will stop all of the different parts of the brain from talking to each other, so there's just one big, rhythmic wave and you can get some sleep! (source)

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ELI5: How, at 93 million miles away, does the sun feel so warm, yet when a simple cloud passes over it the warmth is incredibly dampened?

Answer 1: So when you feel the sun's warmth, you're not feeling heat coming from the sun. You're instead feeling heat created on your skin by the sun's light.Light carries energy. Things with colour, like your skin, absorb light. When they do, the atoms that make them up get 'excited'. Depending on the atom, and what state its in, a few things can happen. If the atom is part of a molecule that energy can go to work breaking it out of the molecule. If the atom or molecule is on the surface of a solid or liquid, the energy can go towards flinging it off, into the air, turning into a gas.If there's not enough energy to do either of those things, then the atom will just release the energy to its surroundings. Most of the time, most of the energy is released as heat. This is what you feel when the sun feels warm. Sometimes the energy can be released as light. This is how glow-in-the-dark things work.A cloud doesn't block all the light from the sun, but it does absorb scatter a lot of it. Think of water droplets in a cloud like a million tiny disco balls. The light that gets through is either too sparce to be noticeable, or high-enough energy that it causes damage instead (ie. UV light). (source)

Answer 2: For all those 93 million miles, the sunlight has had no stuff to interact with. Clouds are very very dense compared to space. A lot of the sunlight interacts with the clouds before it can reach you on the surface, leaving less sunlight to warm you up. (source)

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ELI5: Why is the skin on bald men's heads shiny but skin everywhere else on the body is dull?

Answer 1: Your head secretes an oil called sebum and it still does so even after you lose your hair. Not washing your hair for a few days will make it oily, the same oil makes your bald head shiny without the hair. (source)

All skin does this, but the top of the head does it more. Fun fact: the reason you get acne around puberty is because we used to have way more hair on our bodies but the sebum oil glands never quite got the memo that the hair is far less prominent. The glands are producing excess oil which can clog pores and traps dirt/dead skin/dust (blackheads) or harmful bacteria (white heads)

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ELI5: How does the seat belt know that it's being pulled fast (crash) and stop extending but still extends when pulled slowly?

Answer 1: True eli5: imagine standing in a very small room. Relax your arms and turn slowly. Your arms will stay close to your body. Now turn fast. Your arms will move away from your body and hit the walls slowing you down. (source)

Answer 2: It uses what's a called a centrifugal clutch. Inside the spindle there is a mechanism with 2 grips. When the spindle spins quickly the grips are thrown outwards (just like you are on a merry go round) and stop the belt extending. When you pull slowly the grips don't engage. (source)

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