If I was the kind of person who paid for jokes on a volume per usage basis, this particular joke would be the most cost effective joke of our family. 

Caterpillar: "Knock, knock."

KingofHearts: "Who's there?"

CP: "Banana."

KoH: "Banana who?"

CP: "Knock, knock."

KoH: "Who's there?"

CP: "Banana."

KoH: "Banana who?"

CP: "Knock, knock."

KoH: "Who's there?"

CP: "Banana."

KoH: "Banana who?"

CP: "Knock, knock."

KoH: "Who's there?"

CP: "Orange."

KoH: "Orange who?"

CP: "Wow!  I'm sure glad I didn't say 'banana' again!"

Me:  "You and me both, honey.  You and me both."