Ocean water (1) is highly contaminated at about 35,000 ppm (parts per million) of TDS (total dissolved solids). When the sun (2) heats the surface of the sea water, molecules evaporate from the ocean surface (3) and rise invisibly into the atmosphere to form clouds (4). The clouds retain the water vapor until the air (5) cools sufficiently, and the moisture falls to the earth’s surface as rain (6). Most falls back into the ocean, but some falls on the land (7) where it is collected in lakes, streams and rivers.