From the ArcaMax Publishing, Jokes Newsletter:
http://www.arcamax.com/news/jokes/s-361019-416631
Little Johnny returns from school and says he got an F in arithmetic.
"Why?" asks the father.
"The teacher asked 'How much is 2x3?' and I said '6'"
"But that's right!"
"Then she asked me 'How much is 3x2?'"
"What's the blankety-blank difference?"
"That's exactly what I said!"