When water turns into a gas vapor,(evaporates) it is quite pure.
When it cools up in the air (Condenses), water droplets can form around tiny particles of dust or other material, and many of these together will form a cloud.
Continued cooling increases the condensation, and the water finally falls (precipitation) as ice, snow, or rain.
As gravity draws this precipitation down the water seeks the lowest point it can, almost as if it is burrowing towards the center of the earth. The contours of the land and the kind of material the water contacts decide whether it flows over the land as rivers and streams, seeps into cracks and soil to form "groundwater", or collects in ponds, lakes, or oceans....this is where humans come in, to reconstruct the pathways and collection places for water, and to process or treat it so that it meets our standards of health and flavor.