Knock-knock joke
~noun
Entry: 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 isn't being told repeats the name, followed by "Who?")
(The teller of the joke says a pun based on the name).