Heat from the sun can be harnessed to cook.
There are three broad designs of solar cookers: box, panel and parabolic.
Box cookers are a simple box with a clear lid for light to enter. They may also have reflectors to increase the amount of light entering the box. The light enters the box and raises the temperature in the box, thus heating and cooking anything in there.
Panel cookers work by focussing light onto one spot with a number of flat panels. A pot or cooking vessel is placed where the light is focussed and becomes heated.
Parabolic cookers are convex shaped dishes used to concentrate the energy onto the base of a pot. These cook as fast as a conventional stove. However, they have the disadvantages of high cost and difficulty to make, and the need for focussing frequently. There is some risk of burns and eye damage when used incorrectly.
A good site on solar cooking can be found here.