The scenario is of a person knocking on the front door to a house. The teller of the joke says, "Knock, knock!"; the recipient responds, "Who's there?" The teller gives a name (such as "Noah"), a description (such as "Police"), or something that purports to be a name (such as "Needle"). The other person then responds by asking the caller's surname ("Noah who?" / "Police who?" / "Needle who?"), to which the joke-teller delivers a pun involving the name ("Noah place I can spend the night?" / "Police let me in—it's cold out here!" / "Needle little help with the groceries!").
The formula of the joke is usually followed strictly, though there are cases where it is subverted.
A possible source of the joke is William Shakespeare's Macbeth; first performed in 1606. In Act 2, Scene 3 the porter is very hungover from the previous night. During his monologue he uses "Knock, knock! Who's there" as a refrain while he is speaking:
Knock, knock! Who's there, i' the name of
Beelzebub? Here's a farmer, that hanged
himself on the expectation of plenty: come in
time; have napkins enow about you; here
you'll sweat for't.
Knock, knock! Who's there, in the other devil's
name? Faith, here's an equivocator, that could
swear in both the scales against either scale;
who committed treason enough for God's sake,
yet could not equivocate to heaven: O, come
Writing in the Oakland Tribune, Merely McEvoy recalled a style of joke from around 1900 where a person would ask a question such as "Do you know Arthur?", the unsuspecting listener responding with "Arthur who?" and the joke teller answering "Arthurmometer!"
A variation of the format in the form of a children's game was described in 1929. In the game of Buff, a child with a stick thumps it on the ground, and the dialogue ensues:
What says Buff?
Buff says Buff to all his men, And I say Buff to you again.
In 1936, the standard knock-knock joke format was used in a newspaper advertisement. That joke was:
Rufus the most important part of your house.
A 1936 Associated Press newspaper article said that "What's This?" had given way to "Knock Knock!" as a favorite parlor game. The article also said that "knock knock" seemed to be an outgrowth of making up sentences with difficult words, an old parlor favorite. A popular joke of 1936 (the year of Edward VIII's brief reign) was "Knock knock. Who's there? Edward Rex. Edward Rex who? Edward Rex the Coronation." Fred Allen's 30 December 1936 radio broadcast included a humorous wrapup of the year's least important events, including a supposed interview with the man who "invented a negative craze" on 1 April: "Ramrod Dank... the first man to coin a Knock Knock."
The format is so well known that it can be changed to humorous effect. This is shown in this circa 1980 joke:
Interrupting Cow who? (of course interrupted by an unexpected and loud moo!)
Other variations feature an Interrupting Pig, Interrupting Duck and other equally bothersome animals.
"Knock knock" was the catchphrase of music hall performer Wee Georgie Wood, who was recorded in 1936 saying it in a radio play, but he simply used the words as a reference to his surname and did not use it as part of the well-known joke formula. The format was well known in the UK and US in the 1950s and 1960s before falling out of favor. It then enjoyed a renaissance after the jokes became a regular part of the badinage on Rowan & Martin's Laugh-In.
- Linton Weeks (3 March 2015). "The Secret History of Knock-Knock Jokes". NPR. Retrieved 27 June 2016.
- Henry Bett (1929). The games of children: their origin and history. Singing Tree Press. p. 87.
- Horn, Maurice (1999). The World Encyclopedia of Cartoons. Chelsea House. ISBN 978-0-7910-4855-9.
- "Hee Haw News" p. 4. Rolfe Arrow. (Rolfe, Iowa). 10 September 1936.
- "'Knock Knock' Latest Nutsy Game For Parlor Amusement." P. 1.3 August 1936. Titusville Herald (Pennsylvania). Byline 2 August. New York.
- "Wallis Simpson 'not good looking'". The Daily Telegraph. 1 November 2010. Retrieved 7 November 2015.
- Allen, Fred; Hample, Stuart (2001). All the Sincerity in Hollywood--: Selections from the Writings of Radio's Legendary Comedian Fred Allen. Fulcrum Pub. p. 3. ISBN 978-1-55591-154-6.
- Rees, Nigel (2006). A Word in Your Shell-like: 6,000 Curious & Everyday Phrases Explained. Collins. p. 395. ISBN 978-0-00-722087-8.