"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:
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. 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 first two lines are a complete story by themselves:
The story won the 2012 Cordwainer Smith Rediscovery Award; James Nicoll, however, describes it as "fairly conventional".