You're out in the woods, and you've lost your spare matches. All you've got left to start a fire is water from a stream and a sandwich bag. It'll have to do.
And it will do, as the video above demonstrates.
Grant Thompson, known as the King of Random on YouTube, takes a plastic sandwich bag, fills it up about halfway with water, seals it and begins to twist the bag into a bulb-like shape, with the goal being to get it as close to a sphere as possible. The result, if everything is done correctly, is that the bag and water form a biconvex lens, or, you know, a magnifying glass.
From there, it's something anyone who has toyed with a magnifying glass on a summer day knows: find the most rounded spot in the bag and let direct sunlight shine through it. That light bends and focuses as it passes through the bag and the water until it's a single, narrow line of heat that will set things on fire, like some dry tinder.
Thompson doesn't stop there, though. He has a few more tips on the best ways to start and maintain a fire. Me, personally, I'm not crazy about the idea of spinning around with combustible materials, but I understand why it's a good idea for starting a fire. I'd just worry about setting the whole forest (and myself) on fire in the process.