crack is whack

We have this cute little book called "Eating the Alphabet" by Lois Ehlert and the kids love it.
This morning, I sat with Natalie, reading it to her, cuddling her, and pointing out the letter "B" and pointing to the fruits and veggies. Here is what happens:

me: "look Natalie, blueberries, beans.." (emphasizing the sound of the letter "B") "what else starts with the letter B Natalie?"
James: (chiming in with excitement) "BUTT CRACK!!"
Natalie: giggling

Well, he was right.