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:
Reginald: Who's there?
Reginald: 'Em' who?
George: Emphysema! So you better quit smoking!