Mr. Ratliff's first hour science class built solar ovens using cardboard and tin foil. The purpose of the experiment was to measure the increase of the air temperature every five minutes. There was a zip-lock bag that contained water and that accelerated the rise in air temperature. (I think I got all that correct!) The boys pictured at the end seemed to have the best design, because their air temperature was 110°C at the end of 14 minutes. Amazing!