From the ArcaMax Publishing, Jokes Newsletter:
http://www.arcamax.com/news/jokes/s-367491-451290
A certain professor arrived late for a lecture to find a most
uncomplimentary drawing of himself on the blackboard.
Fuming, he asked the class joker in the front row, "Who, pray, was
responsible for this atrocity?"
The joker won tremendous prestige with his reply, "I really don't
know, but I strongly suspect its parents."