There is a little boy and a little girl in the woods. The little girl asked the boy, "What is a penis?" The boy replied, "I don't know." At that time he hears his mum calling him for lunch. He goes home and eats his lunch. Then he sees his dad on the couch. He goes up to his dad and ask him, "What is a penis?" The dad whips his out and says to the boy, "This is a penis, as a matter of fact this is the perfect penis."
The boy leaves to go find his friend and brings her to the woods. The girl again asks him what a penis is. He whips out his penis and says to her, "This is a penis, and if it was two inches smaller it would be the perfect penis!"
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!"