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The refrigerator operates on the same basic principle as evaporative “swamp coolers” that help keep houses cool in hot, dry climates. The design consists of two cylinders, one inside the other, with a gap between the inner cylinder and outer cylinder. The inner cylinder is made of solid metal. The space between the inner and outer cylinders is filled with a layer of material that can be soaked with water, such as sand, wool, or dirt. The outer cylinder is made of a solid material such as wood or plastic, with holes drilled in the sides to let the sun’s energy reach the wet layer.