Hey, smart people. Joe here. Have you ever heard the phrase "to drink like a fish?" Well, unless you're an ocean-dwelling（住处） species, that probably means staying sober（冷静的）, because freshwater（淡水的） fish don't drink. Saltwater（盐水的） fish, you know their hand? They're constantly（不断地） drinking. But if some fish drink all the time and other fish don't drink at all, we're left with an obvious question.

  Do fish pee? What, you've never wondered that? Water is essential to life as we know it. To keep their chemistry running smoothly, living things need just the right amount of water in their bodies. You'd think that fish have it made when it comes to staying hydrated, but it's not so easy. Fishgills let 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（证明） that with these eggs. I've dissolved（使溶解） the shells away, leaving a membrane that'll allow water to pass through. One is covered in pure（纯粹的） H2O, and one goes into sugar syrup（ 糖浆）. Now, 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（收缩）.

  Life is all about balance, homeostasis. Now, we keep the salts and dissolved chemicals in our blood balanced by flushing（奔流） the extra out through our kidneys（[解剖] 肾脏） and by drinking fresh water when we're thirsty. But ocean fish only have one option. Drink salt water in order to get the H2O, they need to stay alive. And most of them do that a lot, but they need to get rid（使摆脱） of the salt without dehydrating（(使)脱水）, which they do using special cells（细胞） in the gills（鳃） that pump（用抽机抽） out the extra.