One day the teacher walked up to the blackboard and she noticed someone had written the word 'penis' in tiny small letters. She turned around, scanned the class looking for a guilty face. Finding none, she quickly erased it, and began her class.

The next day she walked into the room and again saw, in larger letters, the word 'penis' on the blackboard. Again, she looked around in vain for the culprit, but found none, so she erased the board and proceeded with the day's lesson.

Every morning, for about a week, as arrived at class she found the same word written on the board, only each day's word was larger than the previous day's word.

Finally, one morning she walked in, expecting to be greeted by the same word on the board, but instead, found: 'The more you rub it, the bigger it gets!'