(General talk about) jokes and humour

[Mervi]

I've nothing against stupid humour. Or kid's jokes. But there's usually something you have to figure out about a certain set of jokes. Like once you crack the "how do you know a banana has gone swimming" riddle

Spoiler

the peels are on the beach

you can answer many more "how do you know" jokes.
It's just that I don't "get" the knock-knock ones. I suspect it's a language barrier thing.