The Joke's On...Me?

So yesterday, Noah says to me, "Why is six afraid of seven?" I wasn't sure what he was getting at. Was it a real question? Slight pause, and then it occurred to me that this might be a joke. "What, Noah?" "Why is six afraid of seven?" "I don't know, Noah. Why is six afraid of seven?" "Because seven eight nine." "That's really funny, Noah." "Ha. Ha. Ha," came Noah's reply. Followed immediately by "I don't get it." Noah. In a nutshell.