Water is the most important liquid on Earth. It covers almost 75 percent of Earth’s surface in the form of oceans, rivers, and lakes. All plants and animals need water to live.
Water is made of tiny units called molecules, which are combinations of even smaller units called atoms. A molecule of water is made of two hydrogen atoms and one oxygen atom. The scientific formula for water is H2O.
Water can be found in three physical states: liquid, solid (ice), or gas (steam or vapor). Freshwater boils when it reaches 100° C and freezes at 0° C.
Most of Earth’s water is in the oceans. But water is constantly being recycled. Through a process called evaporation, water moves from Earth’s surface into the air as water vapor. The vapor can form clouds, which make rain and snow. Rain and snow bring water back to the ground again and into lakes, streams, and oceans. Some of the water seeps underground, where it is called groundwater.