Because HE is the one who knocks.
Hilarious ... My little sister told me this and I wasn't expecting it at all. :
Nitrogen Monoxide
A. "It's okay Daddy I'm not hurt."
A: She's got no arms Me: Knock Knock Them: Whose there Me: Not Lucy.
Well it's not Saving Jane, that's for sure.
Because it was on the house.
FREEDOM RINGS
Boris with more knock knock jokes !