I think the most important part of a story is the first 500 words. When someone picks up a book in a bookstore, what do they look at? First, obviously they see the title and author, they read the blurb (often not written by the author), and then they read the first page or two. I often read random pages later on as well.
That first page is the reader's first contact with the author's style (if it is a new author to them). It will have between 250 and 500 words on it, and is likely to be the sole basis upon which the reader will make their buying decision until the author becomes known and/or reviewed.
So here's the challenge. Using the scenario below, write the first page or so of a story (novel or short story - it doesn't matter). That should be around 250-500 words and no more than 1000. It should be the exposition of a story, not necessarily an entire work.
Gareth wakes up in a chamber at the bottom of a 30 foot shaft that he and Ewan have fallen down. Ewan is dead. Gareth has broken his left wrist and badly sprained his left ankle (at least a sprain, possibly broken). There is no way to climb out.