Who’s there?
Cows
Cows who?
No, Cows “Moo,” silly!
And the series starts with this one:
I’m tired so I"m just gonna go with it
Knock knock
Who’s there?
Who
Who, who?
Are you an owl?
Who’s there? 
And I thought this was only funny to a 4 year old…
The only funny knock-knock joke is the ‘Interrupting Cow’ one, and variants thereof. It’s a scientifically proven fact.
Though the Philip Glass one may just be an exception . . .
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In 4e, it’s an ‘Opportunity Action Cow’, of course.
I didn’t get the Philip Glass one. 
Philip Glass is famous for writing very long musical pieces which and uncultured philistine might be forgiven for assuming he was just noodling. For 20 minutes.
IF TIME<= (20 minutes)
THEN
[INDENT]dadadadaDADAdadaddadaDADADAdadada(chord change)dadadaDADADADA(chordchange)dadadadadaDADADADADAdadadada(chordchange back to first chord)dadadada da da da da DADADADA!! da da (chordchange)[/INDENT]
ELSE
[INDENT]da da da DA . . . DA . . . da . . . da . . . da . . . (wait for it) da.[/INDENT]
Knock in this case being and acceptable substitute for ‘da’
–
Basically, the type of music that would be suitable for the soundrack for a documentery about the universe. Written by a new age philosopher who’s absolute lack of understanding in regards tophysics in no way prevents him or her from building thier entire worldview around it. Or an Audi commercial.
-stealing the OP for my stepson’s FB page, lol!
The joke in the OP was a huge hit with my kids. Thank you! I am cooler in their eyes, at least for the next few minutes.
The youngest (my five year old) was inspired to initiate his own knock knock joke:
Him:“Knock Knock!”
Me: “Who’s there?”
Him: “Banana!”
Me: “Banana who?”
Him: “Banana joke!”
He’s very meta, my five year old
I am a fan
OK, so then you tell that one about three to four more times, following with:
Knock knock
Who’s there?
Orange
Orange Who?
Orange you glad I didn’t say banana again!?!
This has been my favorite KK joke for ages. (I was a classical pianist wanna-be many moons ago.)
.
The uncultured philistine would be forgiven because most classical musicians (in my experience) feel the same way.
.
An Englishman, an Irishman and a Scotsman walk into a bar. The bartender looks at them and says “What is this? Some kind of joke?”
Knock knock
Who’s there?
What cheese do you use to entice a bear?
.
.
.
.
.
.
Camembert.
That reminds me of when my daughter was 3 or 4 (she did this for a while) and liked to tell this joke:
Why did the banana go to the doctor’s?
Because it wasn’t peeling well!
Everyone always laughed, because you generally do when a toddler tells you a joke and it’s OK as toddler jokes go.
So she was encouraged, and always followed it up with ‘why did the orange go to the doctor’s?’ because it wasn’t orangeing well!’ which made people genuinely laugh because it was surreal.
It’s a good thing she only knew the names of about 12 fruits at that age.
Knock knock.
Who’s there?
To.
To who?
To whom.