Expand on Fredric Brown’s very short horror story, “Knock”:
The last man on earth sat alone in a room. There was a knock on the door.
Richard Matheson did this, first shortening the story even more by making it one letter shorter:
The last man on earth sat alone in the room. There was a lock on the door.
That turned into I Am Legend, which inspired Night of the Living Dead.
And that’s how Richard Matheson invented the entire zombie genre.