The continuous changes of state and movement of water throughout the Earth system -- the atmosphere, biosphere, lithosphere, and hydrosphere -- is known as the water cycle. The water cycle is driven by energy from the Sun, whose rays cause liquid water from Earth's surface to change into gas in a process called evaporation. As gaseous vapor rises and circulates in the atmosphere, it cools and changes back into a liquid, a process known as condensation. When water vapor condenses, tiny droplets form clouds, which return the water to Earth as precipitation.

Solar water distillation operates on the basic principles of the water cycle -- evaporation, condensation, and precipitation -- to purify water for drinking and other uses. The device used in distillation is called a still, which consists of a basin in which the source liquid is heated, a condenser in which the heated vapor is cooled back to the liquid state, and a collection vessel in which the purified liquid is collected.

As demonstrated in this video segment, a basic solar still can be made using a large bowl, a cover that allows sunlight to easily penetrate to the source liquid in the bowl but prevents water vapor from escaping, and a collection cup no taller than the sides of the bowl. By placing a rock atop the plastic-wrap cover, droplets that form from the pure water vapor will run to the center of the cover. Provided the plastic wrap does not touch the rim of the collection cup, the purified water droplets will fall directly into the cup, leaving any contaminants or unwanted substances in the source liquid to settle in the bowl.

Solar stills have proven to be highly effective in cleaning up water supplies to provide safe drinking water, especially in remote areas or in emergency situations. In hurricane target areas like Florida, solar stills can provide an alternate source of clean water in the event of an extended power outage.

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