The More You Rub The Bigger It Gets

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

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

Little Johnny then screams out back to her, "The more you rub it, the
bigger it gets."