Fruit Salad

We were all in the car yesterday and Molly was in the backseat being a maniac. Out of nowhere from the back we hear:

Molly: Knock knock.
Us: Who's there?
Molly: Banana.
Us: Banana who?

Molly: Knock knock.
Us: Who's there?
Molly: Banana.
Us: Banana who?

Molly: Knock knock.
Us: Who's there?
Molly: Grape.
(Grape??!!)
Us (looking at each other wondering where this is going): Grape who?
Molly: Grape you glad I didn't say banana again? (Followed by hysterical laughter.)

Drew almost had to pull off the side of the road we were laughing so hard. We couldn't decide what was funnier; that Molly has no idea what the point of a Knock Knock joke is or that she thought you could insert any old fruit in place of the "Orange" and that would make the joke funny. This is why they don't let 4 year olds on "Last Comic Standing."