One afternoon after releasing her students from school, a teacher returned to her classroom.  She looked at the chalkboard and there, the day's lessons had been erased and in their place was written the word "PENIS" in distinct, 5th-grader chicken scratch.  Having spent all day looking at her students' handwriting, she knew this wasn't the work of one of her own, and she promptly erased it.

Upon returning in the morning, there once again "PENIS" was written, but just a little bigger than before.  While the teacher erased it she tried to think of the culprit.  The principal *had* winked at her last week after the staff meeting...

That very afternoon, she found "PENIS" on the board again, written still slightly bigger.  She erased it and went to the hallway.  The janitor was just pushing a pile of dust and pencil shavings from the neighboring classroom.  He glanced briefly up at her and leered.  "Please, no...."

She reluctantly opened her classroom door the next morning.  The horror...  "PENIS", written not in small printing, but covering all three slabs of chalkboard in giant letters!!!

Fuming now, still more puzzled and even a bit rattled, she had to stand on a chair to erase from the top of the board.  "Today," she thought, I'll have someone else take the kids down to the buses, and then..."

After lunch, her students at recess, amidst half-finished multiplication tables read:

"The more you rub it, the bigger it gets."