One day when the teacher walked into the classroom, she noticed that someone had written the word 'PENIS' (in tiny letters) on the blackboard.
She scanned the class looking for a guilty face.
Finding none, she rubbed the word off and began class.
The next day, the word 'PENIS' was written on the board again; this time it was written about halfway across the board. Again she looked around in vain for the culprit, so she proceeded with the day's lesson.
Every morning for about a week, she went into the classroom and found the same disgusting word written on the board, each day's being larger than the previous one, and each being rubbed off vigorously.
At the end of the second week, she walked in expecting to be greeted by the same word on the board but instead found the words: "The more you rub it, the bigger it gets."