One student bangs his shin on a table, another student asks "Did that hurt?"
Wincing, the student rubbed his leg and said, "Well, not as much as it hurt the Incas when they did brain surgery without any anesthesia."
This is from a textbook, but I would give massive bonus points if I heard this in my classroom! We've been studying figurative language and idioms, and I've told them if they catch me using an idiom I'll give them a ticket (redeemable for candy or a lottery for privileges); I've also passed out tix for actually using the vocabulary words they study all week, or for identifying a root in a new word I use.
On the other hand, I have told them they will lose every privilege in the book for the rest of the year if they make me laugh while taking a drink, especially when standing over the desk of a classmate, which did almost happen. I can just imagine the parent/principal/never-to-be-employed-student-teacher meeting for that one...