Kidneys are made up of these microscopic loopy tube thingies called nephrons that make sure you have the right amount of salt and water in your blood. They start by dumping out literally everything into the start of the tube except like, the actual blood cells (and some other stuff but I don’t remember that part). Then as the fluid moves through the tube they just pull the right amounts of everything back into the blood and let you pee out the rest.

Most water reclamation is done in a longer tube that’s not as squiggly called the loop of henle. This is because most of your blood is water so you need a bunch of that back. In humans it’s pretty long but in desert animals it’s super extra long because they need a looooot of that water back.

My anatomy and physiology teacher was really good and he used to say there’s basically only two ways we know how or why anything in the body works:

a) it breaks often enough to figure out the differences between functional and nonfunctional parts / systems

b) there’s an animal or plant that can do that thing better, worse, or just generally differently for comparison.

I just tricked you into reading about kidneys, sucker! Now you know how pee is made (and why)!