A common joke, told in the following format: The format does tend to make it annoying, as the second person usually wants to hear a joke, rather than be forced to participate. But it is possible to make them funny in the right situation. Because this trope is so well-known, it's subverted more often than not, by replacing the punchline with an unexpected insult instead. A second subversion is when person 1 isn't actually trying to set up a joke, but person 2 acts like it is. Generally this leaves out the 'knock knock' being said, with person 1 knocking on an actual door.