This is another seemingly popular “knock knock joke provided me by my informant:
Informant: say “knock, knock”
Me: knock, knock
Informant: Who’s there?
[a long pause followed by laughter]
Informant: Yep, that’s the joke!
In this joke, the teller attempts to invert the knock knock sequence, by attempting the get the other party to tell him a joke without having a joke in mind, thus being rendered dumbstruck. It adds an interesting twist of deception to the otherwise predictable “knock knock” pattern.