An elementary school teacher comes into class one morning and sees the word penis in very small print. "All right, which one of you brats wrote this?" Nobody responds so the teacher just erases it.
The next morning the teacher finds a slightly larger word penis on the chalkboard, and erases it.
The next morning there's an even bigger word penis written on the chalkboard, and yet again the teacher erases it.
The next morning there's only a note that says "the more you rub it, the bigger it gets."