A monkey with an apple on his head who fell out of a tree and didn't say banana.
If you don't instantly get the genius of that knock-knock joke, then you've never lived with, or spent much time with, a four year-old. It is one of those universal truths that four year-olds are obsessed with knock-knock jokes and they have absolutely no idea what the rules are pertaining to them. They think it's funny because you say knock-knock. That's all they need. Whatever you say afterwards is irrelevant.
The gem shared above is from my nephew, Marius. We had a joke telling extravaganza the other night and he managed to combine nearly every joke he heard into that amazing punchline. I was instantly put in the mind of my own girls when they were also four years old. They would tell knock-knock jokes incessantly--with none of them ever making sense. They thought it was hilarious. They would laugh until their stomachs ached. They would laugh until they cried. I was delighted to be reminded of that hilarity and get to join in the crazy giggling once again thanks to the most recent four year-old blessing in my life!