Knock, knock.
Who's there?
Fourth of April.
Fourth of April who?
May the fourth be with you!
Alright, listen up, buttercup. This "joke" is a play on words. "May the Fourth be with you" is a famous line from Star Wars, right? But it sounds like "May the fourth" (of April, in this case). So, the joke sets up a typical "knock-knock" format, only to deliver a pun that nerds go crazy over. You probably don't know what Star Wars is but it's ok, just repeat after me 'May the Fourth be with you'.