Try this
 

Tell two regular knock-knock jokes, then:

You: say knock-knock
Them: knock-knock
You: who's there?
Them: (staring blankly)
You: enjoying the mindfuck

;) creative. I like!

Mwa ha ha! I love doing that.... just to screw their heads.

You don't even have to set it up. Just say
"I have a good knock-knock joke. You
start." Works every time.... on children and
software engineers, anyway.

Here's my current favorite k-k joke:

Person 1: "Knock knock!"
Person 2: "Who's there?"
P1: "The interrupting cow."
P2: "The inter--"
P1: "Moooooooooo!"

Cracks me up. Um, if I've forgotten my
medication, that is.

What a treasure. That's actually the first joke I told to the sweet young thing who became my wife....

awesome... thats classic

Applicable in most Fortune 100 settings:

When elephants fight;
The ants get stepped on.

hehhe