Tell me why this joke’s not funny.
Knock knock!
Who’s there?
Banana.
Banana who?
Knock knock!
Who’s there?
Banana. 
Banana who?
Knock knock!
Who’s there?
Orange. 
Orange who?
Orange you glad I didn’t say banana?
It’s a knock knock joke that’s based on a really weak pun, and it’s been so overused.