"Knock" is a science fiction short story by American writer Fredric Brown. It begins with a piece of Flash fiction based on the following passage by Thomas Bailey Aldrich:
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.
It was published in the December 1948 issue of Thrilling Wonder Stories.[2] There have been three different radio adaptations (Dimension X, X Minus One and Sci Fi Channel's Seeing Ear Theatre). The story was reprinted in The Best Science Fiction Stories: 1949[3]