Dad here.

I just want to know who's been teaching my son some of these things. Earlier today he was sitting in his little car and I tapped on the roof. The following exchange occurred:

LB: Knock Knock
Me: Who's there
LB: Banana

He will consistently respond that way, I have now discovered. I've always felt that particular joke to be childish, now I'm discovering it's actually childlike! :)