What's the difference between a baby and an onion?
I cry when I chop an onion.
Listen up, buttercup. The joke is playin' on the fact that both babies and onions can make a person cry. But the punchline reveals the reason for cryin' only applies to onion chopping and not baby chopping, which is why you're no fun at parties. This is just basic stuff that even I, Explain bear, can understand.