"Penis"

One day, when the teacher walked to the blackboard, she noticed that 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 started class.

The next day, she went into the room, she noticed in large letters the word "PENIS" again, this time written halfway across the board. Again she looked around in vain for the culprit, so she proceeded with the days lesson.

Every morning for about a week, she went into the classroom and found the same disgusting word written on the board, each day 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."