I love the scene in Catch Me If You Can when the gmen tell Tom Hanks he doesn’t have a sense of humor. he says, “oh, ya wanna hear a joke.”
Gmen (excitedly): “yeah, tell a joke.”
Hanks: Knock knock.
Gmen: Who’s there?
Hanks: Go fuck yourselves!
I’ve always wanted to try that one out on a kid.
I love that one too.
Also worth trying:
The Interrupting slug (either suck your finger and stick it in their ear, or if you know them really well, grab their head and lick their cheek)
The interrupting tortoise (best delivered after the interrupting cow or others):
Person 1: Knock knock
Person 2: Who’s there?
P1: The interrupting tortoise!
P2: The interrupt… ing… tortoise who?
P1: <sits in silence, as if about to say something>
P2: <waits>
P1: <continues to say nothing>…<makes a few chewing, smacking sounds, as if eating lettuce>… <looks around languidly>
In the “not so much funny as fun playing with people’s minds” vein:
“Wanna hear a great knock-knock joke?”
“Sure.”
“OK. You start.”
“Uh… OK. Knock Knock!”
“Who’s there?”
“uh…”
My fave is an extended version of this.
Knock knock
Who’s there?
Ether
Ether who?
Ether Bunny. Knock knock
Who’s there?
Nutter
Nutter who?
Nutter ether bunny. Knock knock
Who’s there?
Stella
Stella who?
Stellanother ether bunny. Knock knock
Who’s there?
Cargo
Cargo who?
Cargo beep-beep and run over all the ether bunnies. Knock knock.
Who’s there?
Boo
Boo who?
Don’t cry. Ether bunnies will be back next year.
I just tried this one on my wife. It took several minutes before she finally said, “Oh! I get it.”
One of the best yet!
I think my 5yos will love the interrupting cow, but dare not introduce interrupting starfish lest we end up with hands in our faces all day long. They like the jokes, but don’t ‘get’ the idea of puns, so they end up like the ones that Uncommon Sense hears…
Knock Knock
Who’s there?
Muffin
Muffin who?
Muf finger hurts!
And my favourite from 5-7 (I drove my mom batty with this, but I asked her recently and she didn’t remember it…tuning out works!)
Knock Knock
Who’s there?
It’s my underwear!!! 
:rolleyes:
Knock Knock
Who’s there?
It’s my underwear!!!
:rolleyes:
ad infinitum…
I’ve not let my kids in on that one either…
What were the ones in Pursuit of Happyness?
That’s my all-time favorite knock-knock joke.
Somewhere in the middle there should be this one -
Knock knock
Who’s there?
Consumption
Consumption Who?
Consumption be done about all these either bunny’s?
Not a knock knock joke, but in a similar vein,
What are you eating under there?
Under where?
You’re eating underwear? EEWWW!!
I loved this one as a kid.
Tarzan.
Tarzan Tripes Forever.
I’ve told this one in an earlier “Knock-Knock” thread.
Knock Knock.
Who’s there?
Vanilla Ice.
Vanilla Ice who?
That’s show biz.
I was never any good at these. Every time somebody says “knock knock” to me, I just answer “Come in!”
Holy crap did that catch me off guard! Damn near lost a mouthful of water. 
Bravo!
I wish I knew someone I had the balls to try that one on…
I tried the interrupting cow one on my 7-year-old daughter; she was not amused. The conversation went something like this:
Me: Knock knock
Mudgirl: Who’s there?
Me: Interrupting cow
Mudgirl: Interrup
Me: Mooo
Mudgirl: Mom, that’s not polite! You didn’t let me finish!
Me: Yeah, but get it? That’s the joke! It’s an interrupting cow, so it interrupted you!
Mudgirl: You always tell me it’s not polite to interrupt
Me: I know, but that’s the joke
Mudgirl: That’s not funny
Mwahahaha!
I don’t get it.
Knock knock.
Who’s there?
Dyslexia.
Dyslexia how?
My wife has a total soft spot for kid jokes… nothing makes her day more than a bag of laffy taffys.
I am going to torment here.
Here’s one my niece told me last year:
Knock Knock.
Who’s there?
Britney Spears.
Britney Spears who?
Knock Knock.
Who’s there?
Britney Spears.
Britney Spears who?
Oops I did it again.