Knock, knock. Banana.

Q: Knock, knock.
A: Who’s there?
Q: Banana.
A: Banana who?
Q: Knock, knock.
A: Who’s there?
Q: Banana.
A: Banana who?
Q: Knock, knock.
A: Who’s there?
Q: Orange.
A: Orange who?
Q: Orange you glad I didn't say banana?

This joke is another example of a knock knock joke. It has two elements of humor in it - the first, and most obvious one is that they use the word 'Orange' is used as both a fruit and as a pun for the word 'Aren't'. It is a really bad pun, but this is a knock knock joke, so that is pretty much expected.

The other bit of humor is the repetitiveness of saying Banana over and over. Perhaps that isn't very funny to most people, but there is some humor (or perhaps anti-humor) in saying banana so many times.