Stuck in the woods and need to start a fire?
Aside from matches, a lighter, or flint/steel, the sun is a great way to start a fire or heat something up.
You can use a Fresnel lens, which are also used in Concentrated Solar Power (CSP) plants to produce energy. I would explain how they came about, and work, and all that good junk, but wikipedia has an article about it which I linked to above. Obviously something scientifically detailed like the wikipedia article and this one from howstuffworks.c....
Do It Yourself Solar
