One day when the teacher walked to the blackboard, she
noticed someone had written the word ‘PENIS’ in tiny
letters. She turned around and scanned the class looking
for a guilty face. Finding none, she quickly erased
it and began class. The next day, she went into the
room and she noticed in larger letters written across
the board, the word ‘PENIS’. Again she looked around
in vain for the culprit, 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 larger than the previous one. 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.”