The relief teacher walks into her classroom. On the blackboard, she sees a message. It reads, Little Johnny has got the biggest tool in the whole damn school!"
She asks the class, "Who is Little Johnny?"
A kid in the back row raises his hand and says, "I'm Little Johnny."
The teacher snaps, "Well, Johnny, for your very thoughtful and informative message on the blackboard, you're staying after school!"
The next day the relief teacher again walks into the classroom. Litttle Johnny was sitting in the back row with a satisfied grin on his face.
She looks up at the blackboard and there is another message. It reads, "It pays to advertise!"