Thanks I actually do get it now Im 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 knockknock 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 doesnt make much sense for the rest of the joke I also still think the majority of knockknock jokes isnt that funny but thats 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  years from now when doors and identification by speech are obsolete Or even worse what if there are more people like me