knock-knock joke

Etymology
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Noun

 * 1) One of a class of childish jokes told as a dialog, and usually including a pun, in the following form:
 * "Knock knock."
 * "Who's there?"
 * (The teller of the joke says a name.)
 * (The person to whom the joke is being told repeats the name, followed by "who?")
 * (The teller of the joke says a pun based on the name.)