Referring back to my post on making a whistle, the base of the drinks can, can of course be polished to a mirror finish and because of it’s parabolic shape can be used to make fire. Point the can base at the sun and hold a piece of tinder such as fungus, in front of the reflector and adjust the fungus until you find the focal point of the reflected sunlight. The fungus will ignite instantly as you can see below.
Personally I prefer to use the reflector of my torch as it’s something I carry with me all the time, and like the base of the drinks can it is also parabolic shaped. Even a small torch reflector from a Maglite torch, works incredibly well and needs no polishing.