DD is very curious about what's going on in the bathroom - so I let her come in with me sometimes and we talk about what I'm doing, etc. (I know TMI). Anyway. .. when I flush the toilet I say "there's goes the pee pee" and we wave and say "bye bye pee pee" Now every time we walk past the bathroom, Ella stops, waves at the toilet and says Bye Bye Pee Pee.
Funny - slightly gross - but hopefully she won't do it at other people's houses.