The author makes a tacit deal with the reader. You hand them a backpack. You ask them to place certain things in it — to remember, to keep in mind — as they make their way up the hill. If you hand them a yellow Volkswagen and they have to haul this to the top of the mountain — to the end of the story — and they find that this Volkswagen has nothing whatsoever to do with your story, you're going to have a very irritated reader on your hands.
