Where "pee pee" and "poo poo" come from

Mwende was asking me where "pee pee" and "poo poo" come from. So I told her "Let's go look it up on the computer." I threw her question into Google and it came back with this fantastic website, kidshealth.org. So I went through the articles about the urinary system and the digestive system with her. She listened, asked questions and it was generally a fun teaching moment. Later she says "I have to go potty!". Then from the bathroom she shouts "I'm doing my urinary system AND my digestive system!"