Nothing, because he is the one who knocks.
One is white and stinks, and the other is cheese. (As told to me by the UPS guy)
The punchline is too long.
giANT!!
Because she had no arms. "knock knock" "whose there?" "not sally!"
Because HE is the one who knocks.
Heinz-enburg.
because freedom rings. Happy Independence day everybody!
A Jehovah's Witness.