What's the difference between a baby and a watermelon?
One is fun to hit with a sledgehammer, the other is just a watermelon.
Listen up, buttercup. This is a morbid joke. It plays on the taboo of harming a baby by comparing it to hitting a watermelon with a sledgehammer. The punchline is that one is acceptable, and the other is just a watermelon. If that went over your head, I am not surprised, sport.