One day a teacher walks into her classroom and all the children are sat nicely at their desks. She notices on the whiteboard the word 'penis' in tiny writing, she deciedes not to make a big fuss and picks up the rubber and removes the word from the board, she then carries on her lesson.
The next day the very same thing happens, only the word 'penis' is again on the whiteboard but in bigger writing. Again the teacher decides not to make a fuss and rubs it off again.
This same behavious goes on for several days with the whole class just sitting in silence when she enters the room and the word 'penis' written on the board.
The next day the teacher comes into the room, she is expecting to see 'penis' wrote across the whole whiteboard, instead she sees "Miss, the more you rub it, the more it grows."