The Hydrologic Cycle
Water is found throughout the biosphere, and is probably the most important substance needed to sustain lifeforms. Humans can survive for many weeks without the energy obtained by eating food, however, we would only last a few days without water.
Water is used to carry out the many important and complex chemical reactions that all lifeforms must perform in order to survive. Water carries nutrients to various parts of a lifeform and carries waste away from the different parts of a lifeform. Because water is so important, the most abundant substance in any organism is water.
The water found within your body as you read this article has been used and re-used by organisms throughout the history of Earth. It is quite possible that there is water found in your body that was once inside of a Tyrannosaurus Rex, or inside of Julius Caesar.
The process of water moving through the environment is referred to as the hydrologic cycle.