My friend Paul and I bought a 4 foot by 3 foot Fresnel lens this morning. The obvious thing to do with it, of course, is turn it into giant solar magnifier, so we constructed a 1,500 piece K'nex frame for it. By the time we were finished it was 7 in the evening so the sun was fairly low in the sky, but it still got hot enough to vaporize pennies with ease.
I plan to post a video soon when we get a chance to try it mid-day.

The welding mask is necessary to look at the spot, it would blind you otherwise.