Water takes many different shapes on earth: clouds in the sky, waves and icebergs in the sea, glaciers in the mountain, 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 great 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. When appropriately lit, rain takes many colours and forms a wonderful rainbow.
Similarly, water runoffs have played major roles in our history: rivers and irrigation brought the water needed for agriculture and the seas offered opportunity for commerce. Less common forms of runoffs are glacier and waterfalls. Through erosion, runoffs played also a major part in shaping our environment: valleys and river delta hosts many people.
Water also infiltrates the ground and goes into aquifers. This groundwater later flows back to the surface in springs, or more spectacularly in hot springs and geysers. Groundwater is also extracted artificially in wells.
Because water can contain many different substances, it can taste or smell very differently. In fact, we have developed our senses to be able to evaluate the drinkability of water: we avoid the salty seas and the putrid swamps, and we like the water that is adequate for our body.
Water has many unusual properties that are critical for life: it is a good solvent and has high surface tension. Fresh water has its greatest density at 4°C: it becomes less dense as it freezes or heats up. As a stable, polar molecule prevalent in the atmosphere, it plays an important atmospheric role as an absorber of infrared radiation, crucial in the atmospheric greenhouse effect. Water also has an unusually high specific heat, which plays many roles in regulating global climate.
Water is a very good solvent and dissolves many types of substances, such as various salts and sugar, and facilitates their chemical interaction, which aids complex metabolisms.
Some substances, however, do not mix well with water, including oils and other hydrophobic substances. Cell membranes, composed of lipids and proteins, take advantage of this property to carefully control interactions between their contents and external chemicals. This is facilitated somewhat by the surface tension of water.
Water drops are stable due to the high surface tension of water. This can be seen when small quantities of water are put onto a nonsoluble surface such as glass: the water stays together as drops. This property plays a key role in plant transpiration.
A simple but environmentally important and unique property of water that its solid form, ice, floats on the liquid. Its solid phase, is less dense than liquid water, due to the geometry of the strong hydrogen bonds which are formed only at lower temperatures. For almost all other substances, the solid form is more dense than the liquid form. Fresh water is most dense at 4°C, and will sink by convection as it cools to that temperature, and if it becomes colder it will rise instead. This reversal will cause deep water to remain warmer than shallower freezing water, so that ice in a body of water will form first at the surface and progress downward, while the majority of the water underneath will hold a constant 4°C. This effectively insulates a lake floor from the cold.
Life on earth has evolved with and fine tuned itself to the important features of water. The existence of abundant liquid, vapor and solid forms of water on Earth has no doubt been an important factor in the abundant colonization of Earth's various environments by life-forms adapted to those varying and often extreme conditions.