Teacher: "If you're dumb, stand up."
Nobody stands up.
After some waiting, the teacher says, "Really? No one? There must be someone."
Little Johnny stands up.
"Oh, so you think you're dumb, Johnny?"
"Nah, I just feel bad that you're standing alone."
Listen up, smarty-pants. The teacher asks all the dummies to stand, but no one does. Then Little Johnny stands up just to be nice to the teacher so she doesn't feel lonely. He's not really calling himself dumb, just being a pal! It's funny because he's turning the situation around on the teacher, and you thought you were so smart for understanding it.