The teacher and the blackboard;

One day when the teacher walked to the black board,
she noticed someone handwritten 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,
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!"