This is a video demonstrating how to start a fire with nothing but a plastic sandwich bag and some water. It works pretty much exactly how you would imagine, unless you imagined you'd be using elemental sorcery to replace all the water in the bag with fire, in which case I want to live in your brain, it sounds like a magical place.
The intensity of sunlight on Earth is about 1300 Watts per square meter. When you focus the sun's rays using a magnifying glass (or in this case sphere of water) you can increase the intensity roughly ten thousand fold. This increases the temperature of wood to its autoignition point starting the reaction with oxygen in the atmosphere. By protecting the hot embers and adding more energy and fuel, you can get these hot coals to start a roaring fire.
So yeah, if you find yourself lost in the woods by a creek with nothing but the empty plastic bag left over from your last cold-cut sandwich, you could potentially start a fire with it. Alternatively, just poke my smokin' hot bod with a stick. "It just got stuck between two fat rolls." Try it again, my belly thought it saw a marshmallow on the end.
Keep going for the video.
Thanks to Marshall, who agrees if you're lost in the woods, the smartest thing to do is make friends with the bears.