Knock, knock …
Who’s there?
MarkB
boo
Boo who?
Crunch.
Crunch who
Gesundheit.
oh dont cry little baby. 
thank you thank you all so much
Joke #1:
Will you remember me tomorrow?
Yes.
Will you remember me next week?
Yes.
Will you remember me next month?
Yes.
Will You remember me next year?
Yes.
Knock knock.
Who’s there?
See, you’ve forgotten me already!
Joke #2:
Knock knock.
Who’s there?
The Avon lady. Your doorbell is broken.
My favorite (like most knock-knocks, better spoken than written):
Knock knock.
Who’s there?
Interrupting cow.
Interrupting c—
MOOOOOOOOOOOOOOOOOOOOOOOO!!!
My favorite, too.
Children of almost any age get this and love it. They learn from a very early age what interrupting means.
Best wishes,
Jerry
When Tania was five, she made up her own knockknock joke:
Knock, knock …
One of my favourites is as follows:
Me: D’you want to hear a joke?
Other person: Sure!
Me: Say “knock knock.”
Other person: Knock knock.
Me: Who’s there?
Other person: Ummm…
Who’s there?
Nobody.
Knock, knock…
Who’s there?
Bananna.
Bananna who?
Knock, knock…
Who’s there?
Bananna.
Bananna who?
Knock, knock…
Who’s there?
Bananna.
Bananna who?
Knock, knock…
Who’s there?
Orange.
Orange who?
Orange you glad I didn’t say bananna!
I love that one!
Darn branches.
What’s green and has wheels?
…dunno, what??