There are few fears more primal than being cast away on a deserted island in the middle of who-knows-where. Maybe it's a throwback to our eviction from the Garden of Eden, or the nagging realization that Earth itself is just such an island in the endless ocean of space. Whatever the reason, it has been and remains a mainstay of the collective imagination.
You are on a small sailboat in the South Pacific when a freak wave of Biblical proportions swallows your craft. You awake to find yourself on the rocky, sandy beach of--what else?--a tiny deserted tropical island. As the fierce equatorial sun beats down on you, you realize that the boat is gone, but a large section of the white, waterproof nylon sail has washed up on thet beach.
Relieved that you are alive and have sustained no major injuries, you quickly scout out the island. There is a cave for shelter, an abundance of vines and vegetation, but no trees to speak of. You see enough sea birds and marine life to provide a subsistence diet, but there is no source of fresh water! And the rainy season is still months away.
Come up with a reliable way to produce potable water until you are rescued--or the meaning of life is revealed to you, and being rescued no longer matters.
You have only the nylon sailcloth and what you were wearing when you washed ashore: a dark, waterproof windbreaker; a T-shirt; and shorts, in which you find your Swiss Army knife (or Leatherman tool) and a pack of waterproof matches. If it provides additional motivation, feel free to be cast away with the fantasy celebrity of your choice--but this person is still counting on you to provide drinkable water. And if you're looking for extra points here, forget the pack of matches.
Good luck, and rest assured that we're all out there looking for you.