Ice is frozen water. There are many forms of ice. It can be snowflakes, hail or frost. It may form into icicles, and occurs on the Earth's surface permanently as glaciers, pack ice, and ice caps.

Like most pure substances, water can be solid, liquid or gas. Most pure substances will change from gas to liquid to solid, but water can change from gas to solid without being a liquid. This is what happens when frost forms on grass or, for example, on a package taken out of a freezer.

Pure water freezes at 0°C, although if you add salt to water, it will freeze at a lower temperature, commonly 4-5°C below zero. That is why salt is spread on roads. It helps to prevent freezing.

Ice is transparent, although thick ice with air bubbles in, for example in a glacier, absorbs all of the colours of white light except blue, so thick ice appears blue.

Snowflakes grow into delicate star-shaped crystals in the air. In cold air these crystals may fall as snowflakes. When snowflakes settle on the ground, they make a light, fluffy snow. But the weight of snow on the points of the crystals causes the points to melt, and so, quite quickly, the delicate crystal points shorten to thick stumps and take up less space. This is why thick snow gets 'thinner' over a few days even if the air temperature stays below freezing. The water produced by melting crystals quickly refreezes, holding the snow together. This is why the snow stops being fluffy after a few hours.

Billions of ice crystals form in the clouds high above the ground. They occur in most clouds, because it is cold in the tops of clouds. As the crystals grow, they get heavier and fall down into lower parts of the cloud, which are warmer. Then they melt and fall as rain. Most rain that falls starts out as falling ice crystals, even on a summer's day.