When I was an elementary school teacher a part of the curriculum involved learning about energy and the sun. So, recalling the ancient days, I decided to create an experiment where we used sunlight—through a magnifying glass—to cause the ignition of a piece of black paper. Every student was to focus the sun’s raise on that paper until they saw curls of smoke—and then stop. Combining math with this lesson, all students measured the area of the lens and the area of initial incineration (used my callipers for this—area equalling 3.14 times radius squared). Then we divided the area of incineration into the area of the lens. If memory serves, it was about one-six hundredth. So the lens had focussed the energy of the sun into a very small, relative, area—thus ignition.