With a curved mirror (like the mirrored cup from a large flashlight), you can also create a focal point of heat and light. This is a little bit different from light passing through clear lens to be concentrated. With a parabolic mirror from a flashlight, concentrated sunlight forms a blinding white-hot point inside the cup. This means that you’ll need to place the tinder where the flashlight bulb once sat when using a reflecting cup. More shallow concave mirrors (like the bottom of a polished aluminum soda can) will have a focal point a few inches away from the bottom of the curvature. Whichever shape you use, this technique will work the same way. Move the mirror into position so that light focuses on some suitable tinder, and the flammable stuff will start to smolder. The red ember you create can then be blown into flame when combined with more tinder.