A joke made about a caller at the door. It usually ends up as a play on words.

The structure of a knock-knock joke is as follows:

Person 1: Knock-knock.

Person 2: Who's there?

Person 1: (Phrase A).

Person 2: '(Phrase A)' who?

Person 1: (Phrase A) (funny punch line).
Here is a knock-knock joke:

George: Knock-knock.

Reginald: Who's there?

George: Em.

Reginald: 'Em' who?

George: Emphysema! So you better quit smoking!
by Diggity Monkeez May 19, 2005
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Child play for quire little boys on a street corner excepting that they will turn into crackheads later on in life. Most of them have nothing to look forward to.
Come on Paul lets get some knock knock jokes up in dis joint.
by Obama leader of the Democrats December 16, 2014
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