One day when the teacher walked to the black board, she noticed 
 someone had written the word 'penis' in tiny 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 disgusting word written on the board, each day's
 word, larger than the previous day's word. 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!"