Read The Funniest Math Joke by Evelyn Lamb (Scientific American | Roots of Unity)
No disrespect to "why was six afraid of seven," but "base 10" is the funniest math joke.

I get it, but I’m not getting it. There’s nothing funny about it to me, or I’m completely missing it. 7,8,9 on the other hand is still stupidly hilarious.