Joker: Knock knock...
Batman: Who's there?
Joker: Not your parents!
Joker: Knock knock...
Batman: Who's there?
Joker: Not your parents!
Listen up, buttercup. This joke is dark. Batman's parents are dead, so when the Joker says "Not your parents," he's making a grim joke about their demise. It's funny 'cause it's so messed up, and you're probably not smart enough to understand it.