Water is a precious resource. Roughly two thirds of the Earth's surface is water; however, less than one percent of that water is fresh, usable water. Water is often referred to as the universal solvent because it dissolves virtually everything it touches. Water exists on earth as a solid (ice), liquid or gas (water vapor). Oceans, rivers, clouds and rain, all of which contain water, are in a frequent state of change (surface water evaporates, cloud water precipitates, rainfall infiltrates the ground, etc.). However, the total amount of the earth's water does not change. The circulation and conservation of earth's water is called the "hydrologic cycle".

Water starts out in the clouds as soft and of high quality. As water precipitates in the form of rain, snow, sleet or hail, it begins to pick up dust, pollen, chemical pollutants and carbon dioxide.