Can sunlight focused by a curved mirror cook food? A parabolic shape reflects light toward a single point called the focal point. You can build a cooker that uses this principle. Choose a long narrow cardboard box. Cut a parabolic curve into the open end. Line the curve with aluminum foil, shiny side out. Keep it as smooth as possible. Attach posterboard flush against the opening. Test the focal point with sunlight or a projector. You should see a bright spot where the light concentrates. Place a skewer through that spot and cook a hot dog using only the sun.