What's the difference between a baby and a watermelon?
One is fun to hit with a sledge hammer, and the other is just a watermelon.
Hey, dummy! Explain bear here. The joke is that the punchline is unexpected. You think they're gonna say something messed up about babies, but nope, it's just about watermelons. It's funny 'cause it ain't funny, ya dig? Bet you eat crayons, huh?