a four-year-old's humor

Ah, the jokes have begun. Soon enough, she'll have enough to put together an entire stand-up show. Actually, all of her jokes, thus far, are made on-the-fly. Before you read her joke, please note that the hilarity lies in the fact that she finds herself so entirely amusing during the telling.

Q: Why did the balloon go up and up?

A: Because it was full of air!

[Laugh hysterically, causing your parents to also laugh hysterically.]