Water is important because it is essential to life on earth. Humans can only live three days without water, though it is possible to live weeks without food. Water is especially valuable for human health, medicine, agriculture and industry.
The human body is made up mostly of water. Eighty-five percent of the brain is made of water, while blood consists of 80 percent water. Water is vital for nearly every bodily function, including digestion and elimination. Water transports nutrients from foods into the bloodstream, then on to the cells in the body.
A well-hydrated body has good levels of oxygen. Water, after all, is two atoms of hydrogen and one atom of oxygen. The body can't properly burn its stores of fat for energy without oxygen. Lack of water weakens the immune system, and makes a person more vulnerable to disease. Additionally, water is an important component in modern medicine. It is used to clean instruments and for hydrotherapy and dialysis.
Water is also used in agriculture, such as to keep livestock clean and healthy, and to grow crops and produce. It is also used to apply pesticides and fertilizers and to cool crops down when the weather gets too hot. Conversely, water is used for frost control.
In industry, water is used to transport, wash, dilute, cool down and produce materials such as paper and chemicals.