Wondrous Biology

Hopper wakes up from his nap and I ask him if he needs to go potty.

Hop (sadly--sighs): No, I don't have any more pee in my bottom.
Me: Are you sure?
Hop: No, that's why we have to go to the store.
Me: Why do we have to go to the store?
Hop: To buy more pee. For my bottom.
Me: I will tell you a little secret (whispering in his ear),  You don't actually have to go to the store for more pee. You can just drink water.
He looks at me skeptically.
Hop: Are you serious?
Me: I'm serious, you drink water and when it gets to the end of your body, it makes pee.
Hop: THAT'S FANTASTIC!