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 protagonist (the punster) and an antagonist (not in the negative sense; the antagonist here is the recipient of wit). 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.

Some examples

Knock knock
Who's there?
Keith
Keith who?
Keith me, my thweet preenth
(Kiss me, my sweet prince)

Knock knock
Who's there?
José
José who?
José can you see by the dawn's early light?
(Oh say can you see by the dawn's early light? -- first line of The Star-Spangled Banner)

Knock-knock jokes occasionally employ a pun on a common noun or other word.

Knock knock
Who's there?
Orange.
Orange who?
Orange you going to open the door?
(Aren't you going to open the door?)

Knock knock
Who's there?
Boo.
Boo who?
Don't cry; it's only a knock-knock joke.

A similar joke expands the structure:

Knock knock
Who's there?
Banana.
Banana who?
Knock knock
Who's there?
Banana.
Banana who?
Knock knock
Who's there?
Banana.
Banana who?
Knock knock
Who's there?
Orange.
Orange who?
Orange ya glad I didn't say "banana" again?
(Aren't you glad I didn't say "banana" again?)

Another twist on the genre, which was very famous in British school playgrounds for a time, reduced the structure instead:

Knock knock
Who's there?
Doctor
Doctor Who?
That's right!

Widespread knowledge of the knock-knock form gives rise to meta-jokes:

Will you remember me in an hour?
Yes.
Will you remember me in a day?
Yes.
Will you remember me in a week?
Yes.
Will you remember me in a month?
Yes.
Will you remember me in a year?
Yes.
I think you won't.
Yes, I will.
Knock knock
Who's there?
See? You've forgotten me already!

Another twist on the traditional knock-knock format is as follows:

Knock knock!
Who's there?
Impatient cow!
Impatient co . . . (Cut off by next line)
Mooooooooooooooooooooo!!!!!!!!!!!!!!!! Knock knock!
"Who´s there?
Impatient sheep
"Impatient sh . . ." (Cut off by next line)
Moooooooooooooooooooooo!!!!!!!!!!!!!!!

One from Gibraltar:
Knock Knock
Whos there
Ko-po
Ko-po who
Ko-po there a Deq on my Uji?

One from Brooklyn:
Knock knock
Who's there?
Olive!
"Olive who?"
Olive you!

The short film Po Mo Knock Knock is perhaps unique in that the entirety of the film's subject matter is knock knock jokes, specifically postmodern knock knock jokes, as alluded to by the title. For example:

Knock knock
Who's there?
Jacques Derrida
Jacques Derrida who?
Precisely

See Also

• Grape joke