Imagine all human beings swept off the face of the earth, excepting one man. Imagine this man in some vast city, New York or London. Imagine him on the third or fourth day of his solitude sitting in a house and hearing a ring at the door-bell![1]
Fredric Brown condensed this text into "a sweet little action story that is only two sentences long". "Knock" then goes on to elaborate on those two sentences and build a more complete plot around them.