heh heh heh, slurp.
Man, I never thought a "knock knock" joke was going to cause such consternation.
My daughter told me this one, which you are free to use. It has a physical component which I have to explain, but it is hilarious when done correctly.
"Knock Knock."
"Who's There?"
"Interrupting Starfish."
"Interrupting Starfish . . . . "
. . . at that point, you take your outspread hand and either shove it just in front of the other person's face or (if you know them well) adhere it to their face like the creature in "Alien".
Even my boss cracked up at that one.