Knock Knock

My niece came home from her dad's WIRED tonight. "Auntie K! Knock Knock!"
"Who's there?"
"Banana"
"Banana Who?"
"Knock Knock!"
"Who's there?"
"Banana"
"Banana Who?"
"Knock Knock!" She's really getting excited now.
"Who's there?" I blurt out thru laughter.
"Orange!!" She's squealing now that we've finally reached the climax of the joke.
"Orange who?"
"I'm glad I didn't say BANANA!!" She can hardly get the words out thru her hysteria. The joke is "Orange ya glad I didn't say Banana," but the fact that she thinks she's just executed the most perfect laughter evoking joke causes me to double over in tears.