Adventist Youth Honors Answer Book/Camping/Fire/Curved glass
A piece of curved glass can sometimes be used to focus the rays of the sun, igniting the tinder. The glass must be smooth enough to not overly-distort the point of light. A magnifying glass is the typical sort of lens used for this, but there are other possibilities in a pinch.
It is possible, for instance, to use a piece of ice shaped into a sphere to do the same job. Getting the piece of ice round enough and, clear enough, and smooth enough to ignite a fire, however may prove to be difficult.
Others have reported success in polishing the bottom of a soda can, and using it as a mirror to reflect the sun's rays onto a point. Again, it takes a lot of patience to polish the can well enough to be able to light a fire this way, but it has been done. The polishing material can be any fine rouge-like substance, such as clay. The surprising thing about this particular technique is that a chocolate bar has been used as the polishing compound!