One day when the teacher walked to the black board, she noticed someone had written the word 'penis' in tiny small letters. She turned around, scanned the class looking for the guilty face. Finding none, she quickly erased it, and began her class.
The next day she went into the room and she saw, in larger letters,  the word 'penis' again on the black board. Again, she looked around in vain for the culprit, but found none, so she proceeded with the day's lesson.
Every morning, for about a week, she went into the classroom and found the same word written on the board, and each day it was written in larger letters.
Finally, one day, 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!"