A teacher walks into her classroom one day to find the word "penis" written in very small letters in the center of her chalkboard. She looks around to see her student's don't appear suspicious, and erases it quickly, hoping to avoid an embarrassing incident.

The next day, she comes in to find "penis" written again on her chalkboard, only this time larger. She figures one of the janitors must be a pervert or something, erases it, and goes on with class.

This repeats several times, with "penis" showing up on the chalkboard in larger and larger letters. She wigs out on the kids, but none of them know anything.

Finally, she comes into class one day, resolved that if "penis" is written on the chalkboard again, she's quitting. She walked into class, and read the board: "If you rub it, it gets bigger."