Because HE is the one who knocks.
Oh don't worry, they'll tell you.
Nothing, you already told 'em twice.
There's none. They both knock on the door, but never goes in!
Swim down and knock on the hatch. (In Sweden we have a running tradition of telling jokes about stupid norwegians. They do the same about swedes)
Nothing, because he is the one who knocks.
I am the one who Knock-knocks.