The teacher walked into the classroom to find the word "penis" chalked in small letters on the board. She was a bit embarrassed, so she didn't say anything, but rubbed it out and went on with the class.
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!"