You answered the knock knock question with another question...
If the phone rings, do you answer the phone and ask "who's there?" Maybe you do, but that's not the usual response.
I asked a question with knock, knock? You're supposed to answer, not ask a question.
Let's try again. Knock knock?
What is your answer?
Your answer would be Smit.
And I would then ask Smit who?
And you would say gesundheit
ok I was kidding....
So, knock knock, can i speak to Mr Smit?