(Intro)

Stefan: Ahh.  Water.  So good.  I could not live without water.  Seriously, I would die.  We've made a surprising number of videos about water, including exactly how I would die if I stopped drinking it, which we here at SciShow don't recommend.  So here's 2013 Hank to talk about what happens to your body if you go without water.  Spoiler, it sucks, but there's an awesome pee color chart.  

Hank: Just a little while ago we were talking about what your body goes through without food, and I mentioned that a person can live for weeks without eating, and yet we're lucky enough to get a few days without water. So what's the difference?

Part of the reason why we're always dumping more liquid into our bodies is because we basically are water, 50-70% of your body mass is water, and as much as 75% of that bodily fluid is tucked away in your cells, which are, in one way or another, responsible for everything you do.

Your body fluid is mostly water, plus a whole lot of electrolytes which are dissolved ions of minerals like sodium, calcium, magnesium and potassium. Concentrations of each kind of ion charge up the fluid on either side of your cell membranes, and it's that charged separation that your cells use to regulate your body's electrical system, which is what your muscles and nerves use to think and move and break dance and stuff.

In addition to providing this very worthwhile service, your body fluid does a ton of other stuff you need to live. It helps regulate your body temperature by providing sweat, ferries blood cells and nutrients all over the body, keeps food moving through your guts and helps flush out all the leftover waste products. So you can see why, whether you've been chasing a ball in the sun or you have chronic diarrhea or your runaway camel just left you stranded in the desert, it's very important to listen to your thirst.

If you've got cotton mouth, and you're flushed and feeling super thirsty you're probably already mildly dehydrated, meaning your body fluid is running low by about 2%. This first stage is very easy to remedy, just drink a couple glasses of water. But, if you're too oblivious or sick to drink, or don't have access to potable liquids in the middle of the desert, you're going to start sliding off into the next phase: moderate dehydration.

If you pinch your skin and it doesn't snap back right away, it stays, like, tented up there, you are failing the Turgor Test, you're basically starting to shrivel up. You are now missing 5-10% of your necessary bodily fluid. Your head is pounding, you feel nauseated, you're probably hot and confused, your brain is starting to feel all fuzzy. That's because it's mad at you, you're basically draining it's comfy water bed. If you're down by over 10% of your fluids, things are getting dire- you have rolled into severe dehydration.

Your eyes will start settling into the back of their sockets, your pee is gonna look dark and scary, and will probably be painful, if you can even go at this point. Your muscles are cramping, you may start shaking and you'll be feeling super tired and like you're dying a little bit. Mostly because you're dying. Your blood pressure is dropping as your shrivelling cells desperately try to get fluid. You've lost too many of your electrolytes, mostly through sweat and urine, and without the right electrolyte balances your cells can't maintain the voltages they need to activate muscles like your heart. Low sodium levels can also cause your brain to swell.

By now you're well beyond the point of needing a glass of water, you probably need an ambulance, a slow IV of electrolyte fluid and a good amount of luck. And for all of you inquiring minds probably wondering about the value of urophagia or pee drinking: while technically you can drink your own urine at least once, it isn't recommended when you're already very dehydrated. A properly hydrated persons pee is mostly water, but the water-to-waste ratio gets skewed the longer you go without drinking, so a dehydrated persons urine has a lot of waste in it, and by drinking it you're making your already endangered body work harder to filter it all over again, which just isn't worth it, and it's pretty gross.

So, stay hydrated!

Stefan: So you really should drink water, but sometimes when you're trying to be a good water drinker, the world around you can conspire to make it difficult or at least kind of gross.  Here, Michael explains why water has that annoying habit of going stale when you take a glass to bed with you.

Michael: Have you ever poured yourself a nice, cool drink of water before bed and woken up to a glass of blehhhh? In just a couple of hours, your refreshing water turned tepid and kind of musty. But why?

Well, we have a couple guesses about why water goes stale, and it's all about the chemistry. Your drinking water isn't just a pure collection of H2O molecules. It has a lot of other ions and molecules in it too, which can change over time and affect how the water tastes.

So let's first start with carbon dioxide. It's the stuff you exhale, the stuff in soda that makes it fizzy, and that pesky gas that's making the oceans more acidic. And your cup of water is to some extent like a tiny ocean. Water is an excellent solvent, which means it's really good at dissolving lots of other substances, including gases, until it reaches equilibrium with the atmosphere. So as water sits in your room, it gradually absorbs some carbon dioxide from the air. The water molecules and carbon dioxide molecules can then react to each other to form carbonic acid, which lowers the pH of the water, making it slightly acidic. And a little less tasty.

So gas entering the water can affect its flavor. But gas leaving the water can too. Before tap water makes its way to your faucet, water treatment centers sometimes add compounds containing chlorine, up to 4 milligrams per liter, in order to purify it. Chlorine is really good at killing any bacteria or viruses that might be floating around, which is why we also chlorinate swimming pools. But if you've ever accidentally swallowed any pool water, you know it's kinda gross. So then why does a cool glass of tap water taste good? Well, if you're used to drinking slightly chlorinated water, you might associate a tiny bit of chlorination with a cleaner, crisper taste, and when you let a glass of water sit out, the chlorine will dissipate back into the air as a gas, which could change the taste of your water and make it seem less refreshing.

These chemical factors aside, the main reason for the stale taste could be the most obvious: temperature. Basically, cold temperatures suppress taste. Warmer water has faster-moving molecules, which in turn amplify the flavors you can detect with your taste buds. So when you drink a cold glass of water, you might not be tasting all of the subtle flavors it already has inside, kind of like the subtle flavors in a glass of wine.

The good news? Leaving your glass of water out overnight won't hurt you. Even if it tastes a little gross. So: drink up.

Stefan: So if you leave water out overnight, it goes stale.  Well, why not just have a running water fountain next to your bed?  'Cause it makes me want to wet the bed just thinking about it.  But why does running water make you want to pee?  Here's Michael to explain.

Michael:  Listen to this. [water flowing] The sound of a bubbling stream can be pretty relaxing. But, can I ask a personal question...? Do you kinda have to pee now? ‘Cuz if you do, you’re not alone. Just the sound of running water -- whether it’s a leaky faucet, some light rain, or a gushing waterfall -- sends some people running for the nearest bathroom. But why?

Psychologists and urologists, or scientists who study the urinary tract, chalk it up to the power of suggestion. Basically: running water kind of sounds like urination, so just hearing the noise can make you feel like you need to pee. This sort of unconscious association is called a, and the psychological theory behind it has been around for a while. In fact, you’ve probably heard of this classic example -- the Russian physiologist named Ivan Pavlov and his experiments with dogs.

Pavlov would set some delicious meat powder in front of a dog, and it would start salivating -- which is a normal doggy reaction to food, or an unconditioned response. And then, he would ring a bell, and feed it. After repeating this for months, Pavlov could ring the bell without offering any meat powder, and the dog would still start drooling. So his pup had learned to associate a specific sound -- the bell -- with a specific act -- being fed, and began slobbering -- that was its conditioned response.

This same effect could explain our strange peeing urges. Ever since potty training, we learn to associate the sound of running water with urinating and other bathroom sounds like flushing a toilet and washing our hands. So instead of hearing a bell, being reminded of food, and drooling... we hear running water, are reminded of all things bathroom, and need to pee. That’s the theory at least.

The thing is, this phenomenon hasn’t been officially studied in much detail, so scientists aren’t positive that this is the whole cause. But it is a pretty common thing. In fact, doctors have successfully used running water sounds to help prostate surgery patients and people with paruresis [par-you-REE-sis] -- also known as “shy bladder syndrome” -- to turn on the waterworks.

Some researchers even think this power of suggestion isn’t limited to audio cues -- so even just looking at pictures of gushing waterfalls could give you the urge to pee. But that’s enough urine talk for now. We’ll wrap up, in case some of you need to take a bathroom break.

Stefan: We really have learned so much from Pavlov and his pups, even about peeing, but scientists have also learned a lot about drinking water from dogs and cats, including why they drink so differently from each other.  Cats are so tidy while dogs are pretty sloppy but why?  Well, here's Olivia to explain our favorite pets and their weird drinking habits.  

Olivia: You might have noticed how cats delicately lap up milk, while dogs will head over to a water dish and sloppily splash all over your floor. Turns out, there’s some interesting physics to back up the phrase, “cats rule and dogs drool.” That’s because these pets have to use their tongues to get liquids into their mouths, fighting against the effects of gravity. And, according to a couple research studies, cats and dogs have slightly different techniques that do or don’t make a splash.

You and I can sip our milkshakes and soups thanks to suction. And animals like horses, pigs, and sheep can suck up their water, too. And that’s because we all have complete cheeks, which means we have muscles and tissues that let us seal our mouths and create a partial vacuum. Basically, you can make a pocket of lower air pressure in your mouth, so the water you’re trying to drink gets pushed up by the higher-pressure outside air. And that’s suction.

But cats and dogs don’t have cheeks like ours. Like lots of predatory mammals, they have incomplete cheeks, which lets them extend their jaws wider to chomp down on their prey. But they can’t create that partial vacuum and suck up liquid. Instead, they use their powerful tongues to create columns of liquids like water or milk, and lap it up.

Using slow-motion video, different groups of researchers have tried to figure out the physics behind the tongue movements of cats and dogs. One study found that cats lower the tips of their tongues to just barely touch the surface of the water. Then, they quickly retract their tongues at speeds of almost 80 centimeters per second.

The water molecules stick to the cat’s tongue thanks to adhesive forces, and stick to each other because of cohesive forces. And this creates a column of water for a split-second, fighting against the force of gravity.

Dogs, on the other hand, are a little more gung-ho than cats, according to a separate study. These researchers found that dogs splash their tongues deeper into water bowls, curl them into more of a ladle-shape, and then can retract them at speeds of 700 centimeters per second or more.

This means more of a dog’s tongue is touching the water, so dogs end up making a bigger, messier water column before they clamp their mouths shut. The larger the dog, the larger the tongue, and the larger the splash. So next time your dog makes a mess at the water bowl... you can blame physics.

Stefan: Ugh, that's kind of a bummer to learn that dogs can't drink from straws.  How cute would that be?  A little chihuahua sipping its drink with a little pink umbrella!  Oh well, but I bet I know what you're really wondering about: how much water should you drink?  8 glasses a day, right?  Is that still a thing?  Here's Hank to answer that oft-Googled question.  

Hank: How much water should I drink in a day?  Well, it's helpful to keep in mind that you basically are water.  It's your body's principle chemical component.  The average adult is made up of about 65% bodily fluid. And you are that way, because pretty much all of your body’s major systems depend on water to function. Water regulates your body temperature and dissolves minerals and nutrients, and helps carry oxygen to your cells. Your body is what scientists call an aqueous solution - all the chemical reactions that happen inside of you that make you you happen in water.

But still, exactly how much you need depends on who you are, and what you’re up to. Your age and weight and diet and level of activity, and even what kind of climate you live in, all affect your hydration requirements. If you’re running marathons in August, for example, you’re obviously going to need a lot more water than if you’re reading on the couch all day.

But, OK, you’re looking for a number. I get that. You probably heard a lot of them, so you're asking Google. And the guideline you’ve probably heard is that you should drink eight, 8-ounce glasses, or about 1.9 liters of water per day. And no one’s really sure where that advice came from -- even though everyone, up to and including doctors, has repeated it. But according to the Institute of Medicine, the average woman should consume 2.7 liters -- that’s 91 ounces -- of water a day, and men 3.7 liters, or 125 ounces a day.

Now, they point out that you actually get about 20% of your daily water intake from the food you eat -- more than that if you’re eating a lot of fruit and vegetables -- but still, the so-called “eight by eight” approach might not be enough to supply the other 80% In the end, it’s best to use your common sense, and listen to your body. It’s a good idea to keep your eye on your pee -- you want it to be a light yellow color -- but really, just drink when you’re thirsty, and don’t overthink it. Really, don't overthink it.

Stefan: Thanks for watching this watery compilation.  If you have more questions about water or dogs drinking or really anything at all, just let us know in the comments below, and as always, head on over to YouTube.com/SciShow and subscribe.

(Endscreen)