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
 * 2) The recipient: Who's there?
 * 3) The punster: a name
 * 4) The recipient: a name plus who?
 * 5) The punster: the punch line

Quotes

 * [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".