The knock-knock joke is a "question-and-answer" joke, usually ending with a pun. The teller of the joke calls "Knock, knock!", the other person responds, "Who's there?" and the teller gives a name (such as "Boo"). The other person then responds by asking the caller's surname ("Boo who?"), to which the joke-teller delivers a pun involving the name ("Hey don't cry it's alright!!").
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:
- Knock, knock!
- Who's there?
- 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:
- Knock, knock!
- Who's there?
- Rufus who?
- 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 was "Knock knock. Who's there? Edward Rex. Edward Rex who? Edward wrecks the Coronation." Fred Allen's December 30, 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 April 1: "Ramrod Dank... the first man to coin a Knock Knock."
"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 (March 3, 2015). "The Secret History of Knock-Knock Jokes". Retrieved June 27, 2016.
- Henry Bett (1929). The games of children: their origin and history. Singing Tree Press. p. 87.
- "Hee Haw News" p. 4. Rolfe Arrow. (Rolfe, Iowa). Sep. 10, 1936.
- "'Knock Knock' Latest Nutsy Game For Parlor Amusement." P. 1. Aug. 3, 1936. Titusville Herald (Pennsylvania). Byline Aug. 2. 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.