What is bladder ? Pee is one of the first body fluids a kid learn about. You probably learned about pee (also called urine) when you were 2 or so, when you started using the toilet instead of diapers. Now that you’re older, you can understand much more about the amazing yellow stuff called pee. All day long, a small stream of urine trickles down each kidney. It’s collected in an organ called the bladder, which stores the urine until you go to the toilet.