A third-grade teacher came in to the room one day and found a drawing of a penis on the blackboard. She suspiciously looked at her students but didn't say anything. Instead, she rubbed it off.

The next day, she came in and saw another drawing of an even bigger penis on the board. She frowned and rubbed it off.

The third day, she came in and saw another penis drawn on the board. This time, it was huge, covering up almost half of the space.

She couldn't take it anymore, so she screamed out to the room of noisy children, "Why do you kids like drawing this penis on the board? And why is it getting bigger each day?"

Little Johnny screamed back, "The more you rub it, the bigger it gets."