Voldemort: Knock, knock.
Harry Potter: Who's there?
Voldemort: You know.
Harry Potter: You know who?
Voldemort: Exactly!
Alright, listen up buttercup. This joke plays on the fact that Voldemort is often referred to as "You-Know-Who" in the Harry Potter books because people are too scared to say his actual name. So, Voldemort uses this fear in a knock-knock joke. Get it? Probably not, you are not a smart cookie.