On Monday morning, the teacher walked to the blackboard and noticed someone had written the word "penis" in tiny letters. She turned around, but couldn't find the guilty face. She quickly erased it and began her class.
Tuesday, she was again greeted with "penis" on the blackboard, written in larger letters. She looked around in vain for the culprit, and then proceeded with the day's lesson.
Every morning for the rest of the week, "penis" was written on the board in larger and larger letters, and each time, the teacher furiously erased it. By Friday, she'd had enough.
"That's enough," she sputtered. "I -- I can't believe this! Monday morning, I expect an explanation for this behavior!"
On Monday morning, the teacher confidently entered the classroom and found on the board: "Don't you know -- the more you rub it, the bigger it gets?"