Fredric Brown was a master of the short story including what is probably the shortest published science fiction short story:
“The last man on Earth sat alone in a room.
There was a knock at the door…”
The Aswer considers the question “Is there a God?”
I have include the version from “From These Ashes: The Complete Short SF of Fredric Brown” and the version from Baen’s short story collection “If This Goes Wrong” edited by Hank Davis.