Knock knock jokes

Knock-knock jokes are one of the best-known forms of the pun, and represent a time-honoured "call and answer" exercise. Developmental psychologists study knock-knock-joke usage in younger children, as it provides valuable insight into how children advance their language skills.

The standard format has five lines:

1. The punster: Knock-knock (indicating a door has been struck to gain attention)
2. The recipient: Who's there? (an enquiry)
3. The punster: a response, usually involving a name (to set up the pun)
4. The recipient: a repetition of the response plus who? (a request for clarification)
5. The punster: the punch line, which inevitably involves a punnish misusage of the word set up during the response.

Sourced

• [Francisco at his post. Enter to him Bernardo.]
  Bernardo: Who's there?
  Francisco: Nay, answer me: stand, and unfold yourself.

• William Shakespeare, Hamlet, Act 1, Scene 1 (first two lines)
• The play is described in Slings and Arrows as "the longest knock-knock joke in history".^{[specific citation needed]}