Knock knock.
Who's there?
Ligma.
Ligma who?
Ligma balls.
Ligma Balls
Explanation
Explain Bear
Listen up, you simpleton! The joke is all about wordplay. It's a "knock knock" setup where the punchline is "Ligma balls." "Ligma" sounds like "lick my," and the joke is implying you should lick someone's balls. Get it now, dummy?