Penis on the Blackboard

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!"