Speaking of "Knock Knock" jokes...

http://www.dadhacker.com/blog/?p=1024

Sam: Knock, knock!

Me: Who's there?

Sam: Banana

Me: Banana who?

Sam: Knock, knock!

Me: Who's there?

Sam: Banana

Me: Banana who?

Sam: Knock, knock!

Me: Who's there?

Sam: Banana

Me: BANANA WHO?

Sam: Knock, knock!

Me: Who's there?

Sam: Orange

Me: Orange who?

Sam: Orange aren't you glad I didn't say "banana?"