The quality of earth's water remains constant and is billions of years old. It continually cycles around and around in a process called the water cycle. For example the water you used to brush your teeth with this morning is more than four billion years old.

The water cycle has four main processes. These are:

Evaporation - The process by which water moves from the surface of the ocean or the earth into the atmosphere. Heat from the sun evaporates water from the ocean and other bodies of water, such as rivers, lakes and streams. The water vapour rises into the air and is then transported by the wind.

Transpiration - This is the other process that water moves from earth to the atmosphere. Trees, shrubs, grasses and other plants all need water to survive. They take up water from the soil by their roots so that they can grow. Just like in the process of evaporation the sun creates heat which transpires this water into the air through tiny holes in the underside of leaves.

Condensation - Under certain conditions the water vapour stored in the atmosphere begins to form tiny droplets of water vapour. This process is called condensation. The condensed water appears as clouds. Another form of condensation is the formation of water droplets or ice crystals directly on cold surfaces: condensation on ground surface is known as dew or frost. The wind moves the clouds around the atmosphere.

Precipitation - Precipitation occurs when the water vapour cools and becomes too heavy to stay in the sky. As the clouds are moved around by the wind they begin to cool. This turns the warm water vapour into droplets, which results in precipitation. This occurs as either rain, hail, sleet, or snow falling to the ground. The water cycle then begins over again.