Help me figure out a knock knock joke that's had me stumped for 20 years When I was a kid my sisters and I ordered a book of knock knock jokes from the book club at school. I remember reading them to everyone who came by the house but there was one that I never understood. No one in my life has ever had a logical explanation for it and I have never forgotten it: Knock Knock. Who's there? Darth Vader. Darth Vader who? Darth Vader Cookie Crumble. __ WTF?