.
.
[muffled]It’s open![/muffled]
Honestly!
/me sighs heavily
I’ll just play by myself 
“Who’s there?”
“Eat mop.”
“Eat mop who?”
/me giggles uncontrollably
Frederick Frankenstein: “What knockers!”
Inga: “Oh, thank you doctor.”
“Dave’s not here!”
Land shark.
knock knock knock
“Penny?”
knock knock knock
“Penny?”
knock knock knock
“Penny?”
Come and knock on my door
Take the step that is new
Where the kisses are hers and hers and his
Three’s Company too.
Knock three times on the ceiling if you want me.
“Orange you glad I didn’t say ‘banana’ again?”
… Who’s there?
Doctor.
Doctor Who?
… 
… I don’t get it.
MOO!
No, just the Doctor.
Sounds like preignition.
Hey don’t knock it until ya try it.
My recent favorite (not that I’d ever DO it) was one I saw on The Office: (with a German accent)
Knock knock.
Who’s there?
The gestapo.
The gestapo w… (SLAP!)
WE will ask the questions here!
I got a good one- you start it.
Knock knock?
Who’s there?