Knock, knock.
Who’s there?
Panther.
Panther who?
Panther no panth, I’m going thwimming!
[sub]Sorry, folks! My six-year old told me that one and I couldn’t resist![/sub]
Panther Opthinal, Pleath come in.
–bella
[sub]sorry guys, I couldn’t resist[/sub]
Heh. I think I like your answer better, bella!
Knock Knock jokes–a lost art…
I mean, really–if a six-year old thinks it’s funny how can you go wrong? 
–bella
Sure, anyone can understand a six year old’s knock-knock jokes. But it takes a true afficionado to appreciate a three year old’s knock-knock jokes.
I discovered the knock-knock joke when I was about five. Unfortunately I didn’t understand that in joke-telling, the joke-teller knows something that the listener does not know. So here’s me, age five, telling my mom and dad a knock-knock joke:
Me: bursting with excitement Knock-knock!!
Mom and Dad: Who’s there?
Me: Ummm…
long pause, while I finger my lip and look around
Me: Deer!
Mom and Dad: Deer who?
Me: Ummm…
longer pause, while I finger my lip and look around
Me: I 'on’t know!
And they would laugh. And I would think, “Hey, I got 'em!”, and eagerly say, “Knock-knock!” And then my mom would say, “Honey, Daddy has to go to work now…” 
Every day as we pass our neighbor’s house:
Ellen’s children: Mommy! Mommy! Knock, knock!
Me: Who’s there?
Them: Dwayne!
Me: Dwayne who?
Them: Dwayne the bathtub, I’m dwownding!!
Indeed, our neighbor’s name is Dwayne.
This reminds me of that episode of Letterman where he was asked a knock-knock joke by a kid:
Knock-knock.
Who’s there?
Interruping cow.
Interrupting c—
MOO!
Knock-knock!
Who’s there?
Banana.
Banana who?
Knock-knock!
Who’s there?
Banana!
Banana who?
Knock-knock!
Who’s there?
ORANGE!
Orange who?
Orange you glad I didn’t say “banana” again?
Yeah, Creaky, that was a VERY popular one amongst my son and his cousins. But when my son was younger, he didn’t quite get why it was supposed to be funny, and would always do variations on the theme, for example:
Knock, knock.
Who’s there?
Buffalo.
Buffalo who?
Buffalo aren’t you glad I didn’t say banana!
From my Six year old, this one is great for the holidays at the dinner table.
Knock Knock
Who’s there
Noel
Noel Who
Noel bows on the table.
This joke is all the rage with my nieces and nephew (I am proud to say that I taught it to them), and my youngest niece was trying to get in on the fun over Thanksgiving. However, she is not even 2, and thus the fine art of joke-telling is a little beyond her right now:
Her: Knock knock!
Me: Who’s there?
Her: Pay cow!
Me: Impatient cow who?
Her: Door! (cracks up)
Speaking of jokes for the pre-school set, this would just about make me die laughing when I was 4:
Knock knock!
Who’s there?
Mickey Mouse’s underwear!
I never got that joke.
Mudshark, you get it now, though, right? Like “orange you glad I didn’t say banana” as in, “aren’t you glad I didn’t say banana?”
You know, like if you say it fast and kind of slur it, the two phrases sound alike? Um? Okay, it’s a bad joke, but one of my favorites!
I get ot now. Really I do.