Very Interesting!

It’s a photochemical reaction.

There’s a chemical in your skin called 7-DHC that’s photochemically reactive to light between the wavelengths of 290 and 320 nm. These wavelengths are part of the sun’s spectrum and what is often referred to as UVB radiation. When 7-DHC reacts with UVB light, the product is dholecalciferol, also known as Vitamin D3, which is then carried to the liver.

When exposed to UV radiation, epidermal cells in the stratum spinosum and stratum basale convert a cholesterol related steroid into Vitamin D3.

The liver converts D3 into an intermediary product used by the kidneys to synthesize a hormone called calcitriol.

Calcitriol is essential for the absorption of Calcium and Phosphorus in the small intestine.

• In the human skin, we have this steroid molecule (we have innate pathways for steroid biosynthesis) called 7-dehydrocholesterol.
• This 7-dehydrocholesterol, upon exposure to UV light gets converted to a precursor vitamin D3 molecule, which upon isomerization, forms cholecalciferol (an inactive form of vitamin D3).
• Cholecalciferol, in turn, has to be acted upon by 2 enzymes to form the active vitamin D3.
  • 25-hydroxylase in the liver, forming 25-OH-vitamin D3
  • 1-alpha-hydroxylase in the kidneys, forming 1,25-dihidroxycholecalciferol.
  • Both products are active forms of Vitamin D.
• Long story short, sunlight acts as a catalyst in a reaction that converts an intrinsic steroid molecule into usable vitamin D.
• And no, we don't have a special organelle for this. Hope this helped.

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FYI the color of skin is an adaptation to sun exposure in the region of the world your recent ancestors lived. White skin became white in order to absorb more sunlight as we moved north and eventually got trapped during the ice age and the closer your recent ancestors were to the equator the darker the pigmentation in order to protect the skin from the sun. Our hair textures and eye colors adapted the same way. Pretty cool when you think about it and just another why judging some based on skin color is so ridiculous.

Fundamentally this comes down to the "frost line": how far out from the Sun do we need to be before water freezes?

This also depends on the reflectivity of the object in question (the more reflective it is, the less sunlight it absorbs, the colder it is), but generally speaking, it's somewhere in the outer portion of the Main Asteroid Belt separating Mars and Jupiter.

This turns out to be crucial for which planet forms where. A forming proto-planet needs to weigh in around 5 - 10 Earth-masses before it has sufficient gravity to hold onto hydrogen gas. In other words, a proto-planet needs to build up a pretty substantial mass of just solids before it can grow even faster using gas. That is far more likely to happen if you can build your planetary core out of both rock and ice beyond the frost line rather than just rock inside the frost line.

Also note that subsequent migration can then alter planetary positions later. We see a lot of exoplanet systems where Jupiter-sized planets spiraled inwards after formation to end up with an orbit closer than Mercury is to the Sun; we think our own Solar System was spared from this fate only thanks to a resonance between Jupiter and Saturn.

I would challenge your assertion that...

why do we "go back to rocky" when you go even further out from the outer planets?

When you get beyond Neptune, the bodies out there are somewhat rocky, but primarily they're icy. For example, the density of rock is usually somewhere around 3 g/cm^(3), while the density of ice is about 1 g/cm^(3). The density of Pluto, meanwhile, is 1.88 g/cm^(3), suggesting its got a lot of ice. To some extent, you can think of Pluto as a giant proto-planet core that never really gathered enough mass to get started.

This also makes sense because there's more than just one kind of frost line. While we usually means the water frost line just inside Jupiter's orbit, there's also the CO2 line farther out (where carbon dioxide freezes into dry ice), the methane line beyond that, and the nitrogen ice line even farther out than that. So far as we can tell, the objects beyond Neptune have lots of nitrogen ice, because that's just about the only place it can form.

I believe inner planets are more likely to be rocky because the greater solar radiation prevented huge amounts of hydrogen and helium from condensing that close. Out past Neptune, there might just not be enough matter to create another massive gas planet. That's not a complete answer but two explanations that make sense to me

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The sun produces solar winds which push gas outward to a certain point, where it settles enough to accumulate into planets. The solar winds can't push rocks as easily. The larger gas planets usually travel inwards towards the sun and gather up the rocks, too, forming huge planets that are really close to the sun called "hot jupiters." In our solar system, we think Uranus was able to pull Jupiter out of the inner solar system to keep it from eating up the resources that formed the earth.

The puff of air pushes on the surface of your eyeball, and a sensor measures the 'dent' made by the puff of air. This tells them how much pressure is in your eyeball.

(Poke a fully inflated basketball with your finger, the surface doesn't move. poke a deflated basketball with your finger and you can make a noticeable indentation)

It causes the cornea to "squish" slightly under the pressure, and how much of that squish occurs allows the machine to calculate the internal fluid pressure in the eyeball. It's not the most accurate measure around, but it's good enough to tell the difference between regular eye pressure and incipient glaucoma, and since there's no actual contact between the machine and the eyeball, it's an extremely safe means to measure pressure.

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The biggest reason is because when your body starts to prepare for sleep, one of the things it does is it starts producing less tears. This is because it is expecting you to close your eyes and keep them closed for a while, so it shouldn't have to do as much work washing the eyes.

If you push past this and keep your eyes open and stay awake even when your body wants to sleep, your eyes tend to get dry and irritated. At which point closing them lubricates them up again and feels pretty damn good.

The other answers are better but here's one for a 5 year old:

whenever your body needs something, it makes you feel good when you do that thing as a reward! Dehydrated? Water tastes gooood! Super hungry? Just about any food tastes gooood. Time to poop or pee? Feels gooood (usually). Our bodies have evolved this way to make sure things that are necessary are rewarded. When your body needs sleep, it rewards you when you get into a comfortable sleeping position and close your eyes by making that feel goood.

I left out some of the other things that feel good because, well, you're 5.

I'm slightly late but simple evolution/survival advantage usually gives useful biology answers even if you don't know/want the precise mechanisms as others I'm sure will explain.

So closing your eyes/sleeping is useful to do when you're tired biologically sleeping, in the same way it's a good idea to be happy and slightly hungry imagining food/shelter, your body imagining/miming the first step of sleep, which is what it needs when you're tired, should encourage you to do it, it's that simple and for brain and behaviour stuff most things we do give a survival advantage of some kind, although somewhat geared towards a gatherer-hunter lifestyle which can backfire in modern life. Like how closing your eyes during work/school might be considered rude but wasn't so much 10K years ago.

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Why does eating when youre hungry feel good or drinking when youre thirsty? Scratching an itch? Same reason. Brain signals/chemicals rewarding you for fulfilling urges.

Because a small number of companies who have made their business from assisting people with filing tax returns, have paid huge amounts lobbying politicians and making 'campaign donations' (i.e. legal bribes) to prevent it.

They are not allowed to. This is how it works in most other countires. They send you a prefilled tax return based on what they know. You then make any changes to this as there may be a few things the IRS does not know until you tell them. Based on this they will calculate your taxes and send a bill for the remainder unpaid taxes or repay you any overpaid taxes. But in the US there are laws against having the IRS help people fill inn their tax returns in any way. There are IRS employees who have been fired for violating these laws. And even if they were allowed to help you they do not have the funds to do this for everyone, they bearly have enough money to collect and process the data for a few tax returns so they have to trust the information returned in most of them.

Because tax preparers lobby politicians to stop them from making IRS do just that.

This isn’t a joke.

TurboTax and it’s cohorts spend millions each year to make the tax law stay complicated so regular people have to hire them or buy their software to file taxes. Also, TurboTax promises irs to have a free filing system, but they intentionally hide their free filing service from searches.

Lots of countries in the world does exactly what you suggest. When I was working overseas, the government would send me a tax Bill I can just pay online, and I only need to mail something back if I dispute the amount.

The US system is fucked up.

Because I need to pay a meager fee to my non affiliated tax preparer so I can get back as much as I possibly can of the money the government was graciously holding for me without interest.
How else am I going to claim all my work expenses, small business use of my house and the 9 real dolls I claim as dependents. Not to mention their depreciation.

The IRS doesn't know every single detail that could affect how much you owe, so a bill wouldn't always be exactly accurate for every single filer

Many of them are located on places with lax police and get payment through crypto or gift cards.

Pentester here: there's a couple of ways they do this:

1) they require payment in bitcoin. While not totally anonymous, they put it through a tumbler scheme to make tracing much more difficult.

2) they use stolen accounts to pay the money into and withdraw from, meaning the transactions are not associated with them.

3) they reside in countries that couldn't give less of a fuck about extradition requests from the west (eg: China)

They use crypto currency which can be transferred without the police having a way to find out to whom.

As mentioned, by using a form of payment that can't immediately be traced to any one specific recipient, thus at least temporarily evading the eyes of law enforcement.

not sure about regular laundering practices, but crypto is fairly straight forward. just convert it to monero and back. Its private transactions, so its all hidden.

Also stuff like samurai wallet exist to do that with bitcoin and stuff

There are 4 mechanisms of diarrhea. Osmotic, permeability increased, secreting and abnormal motility (peristaltic)

• Osmotic, you have non absorbable substances that impides water to be absorbed and/or causes water to be transfered to the intestine.
  • Artificial swetteners can cause this type of diarrhea (they can't be absorbed, that's why they don't have calories), as well as exocrine pancreatic insuficiency (can't digest fats, that draw water in).
• Increased permeability, means that the intestine, instead of being a semiosmotic barrier as always, becomes more permeable in some way. This is the case in some inflamations and some circulatory diseases with high hidrostatic pressure and transudation.
  • This is basically an edema. The body fighting the infection, increases the vascularity to the zone (more blood flows there), and with some mechanisms, allows electrolytes to go into the lumen.
• Secreting. Normally, the intestine absorbs more (through the villi/mountains) than it secretes through the crypts/valleys (it has that shape in order to maximize absorption). If it doesn't, you are adding oil to a fire.
  • Simple way, if an infection targets the crypts and makes them multiply and continue secreting, it will increase the amount of stuff in the intestine. Or, if it destroys the villi and doesn't absorb, but I guess that would be more akin to osmotic, although they are all conected.
• Peristaltic, instead of having a slow digestion to process everything and absorb all, including water, in an infection your gut goes, "there's something wrong, emergency evacuation" floors the gas pedal, and you drop what you eat as a mishmash that couldn't have time to be absorbed.
  • A day to day example is the difference between eating a lot of fruit [fiber and water and sugars] and red meat [proteins and hard to digest fats]. Also, this motility is the reason why bunnies eat their feces the first time the excrete them, because they can't digest them completely the first time around. (the more you know)
  • (Aclaration: fiber is important for a good digestion. It was the most "extreme normal example" I could think of). Fiber attracts water and accelerates the digestion, being "diarrhea-like", whilst lots of fat, are hard to digest and slow digestion, being "constipation-like". Thus, eating only lettuce, would lead to a diarrhea like poop. Also, the peristaltic mechanism is often secondary to other processes.)

All of them can coexist, and in an inflammation, they usually do, but those are the mechanisms of diarrhea.

TLDR:

Specifically in regards to the virus, it probably will replicate in enterocytes (or some other intestinal cell depending on the virus). It will cause damage there, (decreasing the amount absorbed increasing osmotic pressure if it is in enterocytes [point 1]) and inflamation, increasing blood flow. This will increase the peristaltism [point 4]. In response to the infection, the immune system will respond secreting immunoglobulines and some other things point 3] and it will probably have some damage in the cells that will cause a "leakage" [point 2]. As there is more stuff in the lumen, that won't be absorbed (virus, inmunoglobulines...) there will be an increased osmotic pressure [point 1].

Edits:

1. Changed "types" for "mechanisms" it's more accurate.
2. Aclaration for the fiber part, didn't explain myself too well.
3. Some small changes to ortography and the like
4. Also. Wow, so many votes... And questions. Think I answered most if not all of them.

Certain viruses and bacteria have the ability to activate ion channels that are part of the normal mucosa in the gut. When these are constitutively activated you lose lots of electrolytes and with that, water osmotically follows causing the diarrhea and dehydration.

One of the best ways to explain this is to use cholera as an example. This isn't applicable for all cases but Its a good explanation of the general mechanism.

Cholera excretes toxins that irritate the intestines and cause chloride channels to open releasing a massive excess of chloride ions into the lumen of the small intestine. These ions lower the water potential inside the small intestine (Make the water less wet) so that more water from outside the small intestine moves into the intestine through osmosis so that the water potential inside and out is even.

However, this doesn't work too well in our bodies since very watery stool gets passed quickly and the whole situation gets reset. More chloride ions (now moving with its own concentration gradient because you left the excess behind in the toilet) and therefore more water, therefore more diarrhea.

Your body fixes this by removing the cholera preventing the chloride ions and therefore the rest of the steps.

Another fix that can aid It is Oral Rehydration Salts (dioralyte at the shops) and this has a specific water potential designed to help rehydrate your body and help break this cycle.

Whilst this doesn't apply to all causes it's quite a common mechanism and helps explain how the negative feedback loop can start leaving you to dehydrate and potentially die without your own body or medicine intervening.

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Bacterial infections can cause diarrhea in a number of ways. Toxins, such as cholera toxin, can activate ion channels in your cells and this changes how water flows in your gut, just as u/brainonwheels has said. Cholera toxin activated a chloride channel that causes water to flow out of intestinal cells and into the intestines where it will dilute indigestible materials and leave the body as watery diarrhea.

Some bacteria can change the structure of the gut lining to change how water is absorbed in the intestines and, again, lead to watery diarrhea.

Viruses have even more ways to change gut structure that we are still beginning to understand. Rotavirus, for example, can affect how the nervous system affects intestinal cells, again causing changes to water flow in and out of the gut lining. Norovirus does not affect ion channels in the intestines but seems to change how fast the stomach moves food into the intestines. How norovirus does this at the cellular level is not entirely clear. However, it appears that the immune reaction to the virus causes inflammation of the intestinal lining, leading to decreased water absorption and diarrhea.

TL;DR: Anything that disrupts the normal function of the intestines and how the intestines can absorb water will cause diarrhea.

Edit: formatting

The concentration of salt matters. It's recommended you do sea salt soaks on new piercings to soothe them and help them heal faster. The salt concentration is similar to what's in the body so you don't feel any pain from it.

Depending on what body of water you get into the salinity varies.

Edit: I did not say the sea had the same salinity as the body, just that the salinity level is what makes it hurt or not.

Swimming in the ocean doesn’t make wounds hurt ... ask any surfer. Get reef rash one day ... next day you’re back in the ocean to prevent infection.

I sat there thinking about this for ages. Do fish even know what water is?

No for a few reasons. First, saltwater stings wounds for a little while but not forever. If you have a cut and put it in salt water, the pain will subside pretty quickly. Second, it's debatable whether some sea creatures can even experience pain the way we do because their brains are a lot less developed (obviously this does not apply to sea mammals who have similar brains to use)

The reason sea water hurts is because it has a much higher concentration of salt than in our bodies). Some sea creatures have similar salt concentrations to that of sea water. Others have other mechanisms for removing excess salt or otherwise preventing salt water from irritating exposed tissue like secreting mucous (remember, they evolved in that environment).

Ocean water in cuts doesn't hurt. In fact, I find it makes them less painful and itchy. When it dries though....yowza!

This doesn't answer ALL the factors you asked about but:

• you want your nose close to the mouth to smell your food (for safety reasons)
• you want your eyes as far forward and as high up as possible for seeing things
• You want your mouth far from the waste-hole
• You want all three of those sensory organs as close to the brain as possible (faster processing AND shorter wires are less likely to get injured. This latter one is actually a big deal. If your eyes were away from your brain, breaking your neck would blind you! If your mouth was away from your head, breaking your back means you physically cant eat because you can't swallow).

You're right, the typical configuration isn't the ONLY solution to the above (and other) requirements riddle, but it's a pretty darn good solution - and evolution is very much a fan of "if it's good enough, that's good enough".

Across the animal kingdom, the distribution of sensory organs is a function of the environment and adaptation niche. Accordingly, many of the factors mentioned in early responses give good reasons for the common configuration in vertebrates.

Other groups show great 'creativity' (sorry 😉). Many arthropods (insects, lobsters etc.) smell or taste or hear with their limbs and extremities, for instance, often farther from the brain than even the anus. The more you look, the definition of an 'eye' or 'nose' starts to seem rather fluid and stunningly nebulous. They are often not even localized so you can point when teaching your toddler!

I’d highly recommend reading “Wonderful Life” by Stephen Jay Gould, which focuses on the much wider variety of body plans that existed during the Cambrian Explosion and how a major extinction event radically reduced that variety and led to the body plans we’re familiar with today.

The short answer I'd mention is:

1. we're all digestive tubes with annex systems (evolutionary, that's an important aspect)
2. if the "input" opening is at one end, it's advantageous to have other inputs around that end so you can be more selective with what you try to eat
3. light inputs usually come from above (Sun), so that's where the eyes tend to be arranged. Here's Dawkins explaining it in a nice video: https://www.youtube.com/watch?v=2X1iwLqM2t0
4. other inputs are gathered as you move in a direction and encounter them frontally, so it's again advantageous to have more "input sensing organs" in that region

Now, if you had a mouth at the bottom end, you'd need to live more like a plant and you'd also have to deal with sensing your own waste as it would be near the waste hole.

Lots of invertebrates actually have many visual sensors even on the back of their heads. It's one of the reasons why it's difficult to sneak up on a fly.

A reason could be, that you want to have short pathways to the brain, so you can process information faster.

In higher developed organism, it´s important to know what to eat, so my guess is, that this the reason they are also so close together, and food tends to be on the ground, so the mouth is below the nose and eyes, which otherwise would also get in the way.

Eye position probably developed, because there is a optimal field of view (or other important factor you can prioritize, like the overlapping field of view).

The closer you get to the centre of a black hole, the stronger it pulls, and the faster you need to move to escape that pull.

At some point the speed you need to escape its pull becomes greater than the speed of light. Because speed of light is the fastest speed anything can move with, nothing can escape it. This point is what's called event horizon.

Event horizons are definitionally a boundary beyond which events cannot affect the observer. Specifically, this is a concept that applies to black holes - in an ELI5 sense, imagine a marble in a deep pot with a curved bottom. By swirling the pot around, you can get the marble to climb the walls, but imagine that this pot had sides so steep and tall that no amount of swirling could get that marble out. If one were to imagine that marble is a photon, the highest point you could get the marble swirling would be the event horizon - if anything, marble, spaceship, rubber duck, were to get into the pot beyond that point, it doesn't matter how fast it's going, it's stuck in there. Since light itself can't escape the event horizon, you can't actually see or even detect what goes on beyond that point - and due to relativity (if someone figured out an ELI5 on relativity, that would be entertaining to read) the closer an object got to the event horizon, the slower it would appear to go, so someone watching something go over the edge would just see it slow down to a complete stop as it entered. Black holes are wild.

EDIT: Kurzgesagt came out with a really neat video on this topic a few weeks ago - check it out!

Event horizons are the "edge" of a black hole's...blackness. It's the point where the black hole's gravity becomes so strong that light (and by extension everything else) can't overcome it and gets pulled in, leaving only black nothingness.

This leads to another important aspect about black holes: The actual black hole object (the singularity) is an infinitely-small chunk of mass at the very center, and the hole part that we see is just an illusion, created because all the light that would otherwise travel to our eyes is being sucked into the hole.

Another important thing: If you were to fall into the black hole (assuming you weren't ripped to shreds first), time would pass normally for you, and you would watch the universe shrink away behind you until you were surrounded by darkness and/or ceased to exist. Someone watching you fall in would see a different thing though. They would watch your descent slow down the closer you got to the event horizon, because the light "image" of you falling would become more and more distorted the closer you got. This would continue until you froze in-place completely, at which point you would just kinda fade away since the "image" of you falling in, also falls in with you.

And one last important thing: Most black holes have event horizons a few dozen miles wide at most. But some, at the core of very bright galaxies, are orders of magnitude larger than the largest stars, and could swallow solar systems whole.

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Its the place where information can only be pulled farther into a black hole. Time, space, and light, pretty much immediately lose any relevance of what they were before crossing this boundary. In our observations of what we think is a black hole, we are really only seeing the smear left over by the stars and gas and matter heating up as they approach the horizon.