What is water?

Water (from the Anglo-Saxon and Low German word, wæter) is a colourless, tasteless, and odourless substance that is essential to all forms of life that we know of.
There is a lot of water on our planet, and it exists in many places and forms: mostly in oceans and polar ice caps, but also as clouds, rain water, rivers or freshwaters. Water is continuously moving through the cycle of evaporation, precipitation, and runoff, back to the sea.
All known forms of life need water. Humans consume "drinking water" - water which has qualities compatible with the human body. Ordinary rain water in many countries is polluted and therefore not safe to drink. This natural resource has become scarce with the growing world population, and its availability is a major social and economic concern.

Importance

Water takes many different shapes on the earth: water vapor and clouds in the sky, waves and icebergs in the sea, glaciers in the mountain, and aquifers in the ground, to name but a few. Through evaporation, precipitation and runoff, water is continuously flowing from one form to another, in what is called the water cycle.
Because of the importance of precipitation to agriculture, and to mankind in general, we give different names to its various forms: while rain is common in most countries, other phenomena are quite surprising when seen for the first time: hail, snow, fog or dew for example. In many African countries, snow is, for example, a very rare phenomenon. When appropriately lit, water drops in the air can refract the beautiful colours of a rainbow.
Similarly, water runoffs have played major roles in our history: rivers and irrigation supplied the water needed for agriculture. Rivers and the seas offered opportunity for travel and commerce. Through erosion, runoffs played a major part in shaping our environment providing river valleys and deltas which provide rich soil and level ground for the establishment of population centers.
Water also infiltrates the ground and goes into aquifers. This groundwater later flows back to the surface via springs, or more spectacularly via hot springs and geysers. Groundwater is also extracted artificially from wells.
Because water can contain many different substances, it can taste or smell very differently. In fact, we have developed a way to evaluate the drinkability of water: we avoid the salty seas and the putrid swamps, and we like the fresh pure water of a mountain spring.