A solar still distills water with substances dissolved in it by using the heat of the Sun to evaporate water so that it may be cooled and collected, thereby purifying it. They are used in areas where drinking water is unavailable, so that clean water is obtained from dirty water or from plants by exposing them to sunlight. Still types include large scale concentrated solar stills and condensation traps. In a solar still, impure water is contained outside the collector, where it is evaporated by sunlight shining through a transparent collector. The pure water vapour condenses on the cool inside surface and drips into a tank. Distillation replicates the way nature makes rain. The sun's energy heats water to the point of evaporation. As the water evaporates, its vapour rises, condensing into water again as it cools. This process leaves behind impurities, such as salts and heavy metals, and eliminates microbiological organisms. The end result is pure (potable) water. History Condensation traps have been in use since the pre-Incan peoples inhabited the Andes. In 1952, the United States military developed a portable solar still for pilots stranded in the ocean. It featured an inflatable floating plastic ball, with a flexible tube in the side. An inner bag hangs from attachment points on the outer bag. Seawater is poured into the inner bag from an opening in the ball's neck. Fresh water is taken out using the side tube. Output ranged from to of fresh water per day. Similar stills are included in some life raft survival kits, though manual reverse osmosis desalinators have mostly replaced them. Today, a method for gathering water in moisture traps is taught within the Argentinian Army for use by specialist units expected to conduct extended patrols of more than a week's duration in the Andes' arid border areas. Methods Pit still A collector is placed at the bottom of a pit. Branches are placed vertically in the pit. The branches are long enough to extend over the edge of the pit and form a funnel to direct the water into the collector. A lid is then built over this funnel, using more branches, leaves, grasses, etc. Water is collected each morning. This method relies on the formation of dew or frost on the receptacle, funnel, and lid. Forming dew collects on and runs down the outside of the funnel and into the receptacle. This water