Little Johnny

One day, when the teacher walked to the black board, she noticed someone had written the word "penis" in tiny letters. She turned to the class, scanned the boys and girls, looking for the guilty face. Finding not a guilty face in the bunch, she quickly erased the blackboard and began her class.
The next day, the teacher went into the classroom and noticed, in larger letters this time, the word "penis" scrolled on the blackboard. Again, she looked around the classroom in vain for the culprit, but found none. And so, the teacher erased the blackboard and proceeded with the day's lesson.
Every morning, for about a week, the teacher went into the classroom and found the same word written on the blackboard, each day, scrolled larger than the previous day.
Finally, one day, the teacher walked into the classroom expecting to be greeted by the same word on the board. Instead, she found scrolled on the blackboard:
"The more you rub it, the bigger it gets!"