What's the difference between onions and babies?
I cry when I cut onions.
What's the difference between onions and babies?
I cry when I cut onions.
The humor comes from the unexpected comparison of onions and babies. The setup leads you to expect a gross or shocking answer but it ends up being something very mundane and relatable. The punchline plays on the common experience of crying while cutting onions, creating a funny contrast with what people might expect as the 'difference'.