This all happened while I was driving, so my objective was to shush her as quickly as possible. As you can tell, I failed.
Lily: Mommy, my tummy hurts when I do *this* (bends sideways)
Mommy: You probably just have an air bubble or something baby.
L: What's that?
M: Well, it's a bubble... Made of air.
L: What's inside it?