A little joke

Sofi's learning the art of the knock-knock joke, and Erik must have been listening to her, since he's now come up with one of his own:

"Knock-knock." (He generally pounds on something when he says it.)
"Who's there?"
"Banana." (Or, more accurately, "nana," but we know what he means.)
"Banana who?"
"Cracker."

That's it. He's repeated it several times. We don't really get it, but we do laugh every time -- and I suppose that is the point!