"Water is the driving force of all nature."
(Leonardo da Vinci)

In an animal or a plant, 99 molecules in 100 are water… An organism is a pool in a stream of water along with metabolites and energy move through ecosystems.
(W.V. Macfarlane)

 At any single moment, 94% of the earth’s water is found in the oceans, 4% is in inaccessible aquifers, and 1,5% is locked up in polar ice and glaciers. This leaves about 0,5% of the earth’s water available for human use, but most of this water is hard to reach and much too costly to be of any practical value.
 Three out of every five people in the developing nations do not have access to clean, disease-free drinking water. According to the World Health Organization, 80% of all disease in these countries results from the contaminated water that people drink and bath in.
 Water shortage arises from two principal problems: First, water is not evenly distributed across the face of the earth. Tropical rain forests are drenched with rain whereas much of the US deserts receive less than 25 cm a year. The second reason is that human civilization has pushed well beyond the earth’s carrying capacity, exceeding the readily available supply of this renewable resource, even in the areas with abundant rainfall.
 One of the methods, how to increase water supply is saltwater desalination, the removal of salts from seawater and brackish (slightly salty) water. The two main methods are evaporation and reverse osmosis. In evaporation, or distillation, the salt water is heated and evaporated, leaving behind the salts and minerals. The steam produced is then cooled, and pure water condenses out. In reverse osmosis, water is forced through thin membranes whose pores allow the passage of water molecules but not the salts and minerals.
 Since 94% of the water on earth is in the oceans, desalination might seem like the best answer to water shortages. Unfortunately, water produced by desalination is four to ten times more expensive than water from conventional sources.
 We can also do many things to cut down on water use. For instance: in showers - by installing flow restricters or taking shorter showers, in toilets - by installing low-flush toilet, in faucets - by installing low-flush faucets or flow restricters.