The water cycle involves water evaporating (turning into a gas), rising to the sky, cooling and condensing into tiny drops of water or ice crystals that we see as clouds, falling back to Earth as rain, snow or hail before evaporating again and continuing the cycle. Learn more about the water cycle.
Water in the form of ice is found at the polar ice caps of the planet Mars, some scientists have also suggested the possibility of liquid water on the red planet.
While most people know that water boils at 100 °C (212 °F), this is at the normal conditions of sea level. The boiling point of water actually changes relative to the barometric pressure. For example, water boils at just 68 °C (154 °F) on the top of Mount Everest while water deep in the ocean near geothermal vents can remain in liquid form at temperatures much higher than 100 °C (212 °F).
Water expands as it cools from 4 °C to 0 °C (above 4 °C it does the opposite). In freezing conditions, water has been known to burst water pipes as it freezes to ice.
Most people around the world have access to clean drinking water but it is a major problem in poorer areas of the world. Water pollution and low quality water can lead to dangerous bacteria, disease and viruses such as E coli and Cryptosporidium.
Drinking water is needed for humans to avoid dehydration, the amount you need each day depends on the temperature, how much activity you are involved in and other factors.