I had a friend in grad school who wanted to build a giant parabolic reflector for Burning Man. The idea was to lay the thing out on the desert floor and then be

I had a friend in grad school who wanted to build a giant parabolic reflector for Burning Man. The idea was to lay the thing out on the desert floor and then be able to melt gold, or something, at the focal point so that they could trade sun-melted trinkets with passersby. Never made it to the prototype stage. After watching the Mythbusters try this trick, I'm kinda glad the poor guy didn't spend the time. Fresnel lenses. Better alignment. Much more reliable.