The knock-knock joke is a type of joke, probably the best-known format of the pun, and is a time-honoured "call and answer" exercise.
It is a roleplay exercise, with a punster and a recipient of wit.
The standard format has five lines:
1. The punster: Knock, knock!
2. The recipient: Who's there?
3. The punster: a response, sometimes involving a name (Doctor)
4. The recipient: a repetition of the response followed by who? (Doctor who?)
5. The punster: the punch line, which typically involves a punnish misusage of the word set up during the response