You’d think that fish have it made when it comes to staying hydrated, but it’s not so easy. Fish gills lets oxygen flow into their bodies, but they’re also letting water constantly escape. Ocean water is *saltier* than the insides of fish. So water wants to move from the inside out to balance that out. That’s a problem if you’re a fish who wants to stay alive. It’s a process called osmosis, and we can demonstrate that with a simple experiment . You need 2 eggs, Dissolved the shells away, leaving a membrane that’ll allow water to pass through. One is covered in pure H2O. And one goes into sugar syrup. The syrup contains lots of sugar molecules, and not much water, so water passes out of the egg to balance it out, and the egg shrinks! Life is all about balance. Now, we keep the salts and dissolved chemicals in our blood balanced by flushing the extra through our kidneys, and drinking fresh water when we’re thirsty.