Gareth's Magic Bladder

Gareth: "I peed in the potty and my pee was blue."
Mom: "Oh really?  How'd that happen?"
Gareth: "It just came out of my bladder because my bladder is a magic bladder."
Mom: "It's magic?  What else can it do?"
Gareth: "It can make yellow pee or red or green or even purple pee!"
Mom: "Umm, wow!"

(No, you don't have to worry...there was no actual blue pee.. he made it all up!)