I heard this one two days ago:
Joker: Knock, knock.
Jokee: Who’s there?
Joker: An impatient cow.
Jokee: An impa . . .
Joker (bursting in): Moo!
Why on God’s green earth do I find this so funny? I’m laughing now just thinking about telling it. 
RR
(It works great with a duck, too.)
I read the thread title and THAT was the first joke that came to mind, except I’ve heard it:
Joker: Interrupting Cow
Jokee: Interr –
Joker: MOOO!
You’re right, it’s dumb, but it’s a classic.
That’s a slight variation on the bit that made Johnny Carson laugh so hard I was sure he was going to have a coronary, right there on television.
Buddy Hackett (I believe it was him): Johnny, I’ve discovered the key to all great humor.
JC: Oh, what’s the key to al—
Buddy Hackett: TIMING!
I first heard the interrupting cow as a closing joke on an episode of The Vicar of Dibley.
I always heard it as the “impatient chicken” who gives a really loud squawk.
Somehow, I just can’t moo as loud as I can squawk, and the louder the funnier, IMHO.
huh huh huh
huh huh huh
huh huh huh
Oh I am cracking up. Those stupid jokes are so me!
My favorite dumb jokes
Why’d the crazy man call his son Joe?
a. That was his name
What’d the famer say when he lost his tractor?
a. Where the hell’s my tractor?
heeheehee
You asked for it.
Did you know deer don’t have uncles?
They just have "aunt"lers.
What’s black and white and green and white and black and smells terrible?
Two skunks fighting over a pickle.
What’s the difference between a tiger and a well-dressed man?
A well-dressed man wears a 3-piece suit and a tiger… just pants.
::d&r::
OMG! That’s hysterical! I just spit all over myself!
Really! I’m gonna go tell my husband right now.
Told him the cow joke, and he laughed. 
Told him the skunk/pickle joke, and he groaned.
Told him the farmer/tractor joke, and he just rolled his eyes.
I love the cow joke. Can’t wait until my kids get home so I can tell them. They’ll LOVE it!
Why did the frog cross the road?
Because he was stapled to the chicken’s but.
Why did the pervert cross the road?
To get to the chicken.
Why did the monkey fall ouyt of the tree?
Because it was dead.
Here’s one I remember from elementary school:
Q: What’s round, two inches across, has blue hair, weighs 10,000 pounds and goes “peckety-peck-peck?”
A: Nothing.
Stumps 'em every time.
RR
'Cause that was his name. Waaaahahaha! I can’t take it.
A priest, a rabbi and an indian walk into a bar and sit down.
The bartender comes over and says, “What is this, a joke?”
Me: Ask me if I’m a banana.
You: Are you a banana?
Me: No.
Q:Whats the difference between a grape and an elephant?
A:Grapes are purple and elephants are grey.
Q:What did Tarzan say when the elephants came over the hill?
A: Look Jane here come the elephants.
Q:What did Tarzan say when the elephants came over the hill wearing dark glasses?
A: Nothing he didn’t recognise them.
Q: What did Jane say when she say the elphants coming over the hill?
A: Jesus fucking Christ Tarzan! A bunch of grapes wearing dark glasses is coming over the hill! (she was colourblind).
I predict that this thread is going to go to 3 pages, easy…
You have to be sure to space them out a bit and not tell them too close together or you’ll ruin the effect of the 2nd one you tell…
Q:What is a foot long and slippery?
A: A Slipper!
Q:What is brown and sticky?
A: A Stick!
(it helps to shout the answer and laugh uncontrollably afterwards - at least that is what I always do…)
Q:Why are elephants Big, Gray, and Wrinkled?
A:Because if they were Small, White and Smooth they’d be Aspirin.
So these two penguins were sitting in a tub of rocks wearing blonde wigs.
The first penguin says to the second penguin, “Please pass the ketchup.”
The second penguin gives the first penuin a look and says, “What do I look like, a typewriter?”