The Harder you Rub – Bigger it Gets

But the next day when she came in, she found the same thing again – “penis”, this time written slightly larger. So she rubbed it out again, and went on with the lesson.
Again next day, in larger letters, there was the word “penis” again. With a red face she rubbed it out and went on with the lesson.
Well, this went on for a whole week, every day the word penis getting bigger.
Finally, on Friday she went into the classroom to find chalked up: “See, the harder you rub it, the bigger it gets!”