I wrote a post a couple of years ago about a guy who built a solar death ray using 5,800 small mirrors to focus sunlight onto a 1-centimeter-square spot. His death ray burned concrete, melted metal and baked mud.
Grant Thompson, the self-proclaimed King of Random, recently built his own solar death ray using a light-focusing Fresnel lens scavenged from an old rear-projection television. The Fresnel lens was used inside the TV to help focus the light beam that "draws" the image on the screen. Grant mounted the lens on a swivel and built a holding platform at its focal length. Voilà — instant solar death ray.
Check out Grant's death ray in action in the videos below. The first one shows how he built it, the second just shows him burning stuff. Both are awesome.
Grant has a lot of really great videos on his YouTube channel. Check out his episodes about how to make an emergency phone charger, how to start a fire using a water bottle, and how to make your own electrolysis water fuel converter.