  :::Thanks, I actually do get it now. I'm thinking I probably somehow never made the connection to doors because where I live people speak Dutch, so I would have never been in a (literal) knock-knock situation (in English). Also, when the joke is told on the internet or TV from an English source, there never is an actual door and the people can even see each other. It now does feel stange to have missed it, probably a lack of imagination. I possibly guessed the knocking was knocking on the other persons head, even though that doesn't make much sense for the rest of the joke. I also still think the majority of knock-knock jokes isn't that funny, but that's just a matter of sense of humour.  :::I have actually added a bit to the article to explain it. What if somebody reads wikipedia trying to figure out this joke 500 years from now when doors and identification by speech are obsolete? Or even worse, what if there are more people like me?  