Knock-knock

Xander has been telling "knock-knock" jokes for a while (really the same one that he learned from Dora):
Knock-knock!
Who's there?
Boo!
Boo-hoo?
Don't cry, it's only a joke!

Duncan, imitating his brother, made up his own, borrowing the character of Boondai (Xander's imaginary friend):
Knock-knock!
Who's there?
Boondai!
Boondai-hoo?
Whoooooooo!

He just cracks himself up and tells it over and over. He especially prefers if Xander does the responses.