Ahh, the subtle mysteries of the knock-knock joke.

[wrenlet]

Donovan: Knock knock!
Greg: Who's there?
Donovan: Donovan!
Greg: Donovan who?
Donovan: Donovan [last name]! *cackles madly*

G: Knock knock!
D: Who's there?
G: Apple.
D: Apple who?
G: Knock knock!
D: ... who's there?
G: Apple!
D: Apple who?
G: Knock knock!
D: Noooo, I'm talking about apple who?*
G: It's okay, it'll be funny! Knock knock!
D: ... who's there?
G: Orange!
D: Orange who?
G: Orange you glad I didn't say apple?
D: *laughing* It's orange's last name!
Me: *trying not to snicker*

D: Knock knock!
G: Who's there?
D: Donovan!
G: Donovan who?
D: Donovan's gonna eat an apple!
G and me: *die laughing*

* Standard response when he's fixated on a subject and/or doesn't want to admit he's in trouble.