Water's natural change of forms is called the Hydrologic Cycle. This cycle includes evaporation (water going into the air); condensation (forming clouds); precipitation (rain, snow, sleet, and hail); runoff (as rainwater flows into surface sources); and percolation (water soaks into the earth to form pockets of groundwater).

The molecules of water keep moving from one place to another, sometimes in the air as vapor, sometimes as liquid and sometimes as ice. Without this cycle, people and most living things could not survive on our planet. Here in the desert, we sometimes have a form of precipitation that starts out as rain but evaporates before it reaches the ground. This is called "virga."

Redrawn after Gabler et. al., 1999

Water takes in all sorts of materials as it moves along in its cycle. There is a large space between the atoms, so water can take in or dissolve practically anything. This is why we can use water for cooking, beverages and cleaning, and why it is called the universal solvent.

Since there is no new water forming on our planet, we must reuse what we have over and over. Every time we use water, it absorbs substances called pollutants (such as soap in the dishwater). For millions of years, nature did the trick of removing pollutants slowly as the water moved through its natural cycle.

Today, with all the people on Earth, all using water, we cannot wait for nature to do this work anymore. Besides that, we use many more chemicals that can get into the water. People have had to develop ways to clean water faster, which is called "Water Treatment."