Nature makes extremely pure water through a process called the Hydrologic Cycle. In this process, the Sun provides heat to the Ocean, which causes evaporation. The pure water vapor rises until it condenses into clouds. With further cooling the water in the clouds precipitates as rain. Rain, as long as there is no air pollution, is extremely pure water.

Steam-distillation exactly duplicates this process. In steam-distillation, the contaminated water (your tap water) is boiled and evaporated. The pure water vapor rises (steam), then further cooling condenses causing it to precipitate into fresh water droplets.