The document explains how rain forms through the water cycle. It states that rain is the primary source of fresh water and provides suitable conditions for ecosystems. The water cycle involves the sun heating surface water, which evaporates into water vapor in the atmosphere. The water vapor condenses into liquid water droplets as it cools, becoming heavy enough to fall as precipitation like rain or snow. However, some rain evaporates before reaching the surface in hot, dry regions in a phenomenon called virga.