Water becomes ice when it is very cold. Liquid water freezes and becomes solid ice at a temperature of 0° Celsius (32° Fahrenheit or 273 kelvin).
Ice is the common name for frozen water. Other liquids (say ammonia or methane or milk) could be called ice when they freeze but we would always say 'milk ice', for instance, instead of just 'ice'.
Places like Antarctica have lots of ice, but people can also find it in home appliances (machines in the home) like the refrigerator or freezer. If people put water in a freezer and leave it for a while, the water gets very cold and will freeze solid, creating ice. People can put water into a copper (or metal) container if they want ice to freeze faster. Copper is a very good conductor of heat--it can freeze water faster than a regular (plastic) ice tray would be able to. Surprisingly, an open tray of hot water can freeze faster than the same amount of cold water! This happens because enough of the hot water can evaporate before cooling, reducing the amount of water to be frozen.
Unlike other liquids, water expands as it freezes to become ice; so ice floats on water because ice has less density than water. This is very unusual - just about every other liquid gets more dense as it cools; water ice, however, is an important exception. Liquid water expands by about 9% as it becomes ice - it takes up more space. If water in pipes freezes it can burst the pipe. Water in glass bottles can explode in the freezer if people leave it there long enough to freeze. Water freezing in rock crevices can expand enough to split hard rocks apart; this is an important geological weathering process that can wear down mountains and make rock into soil.
Salt water needs a lower temperature to freeze than pure water. The resulting ice contains much less salt than the salt water it came from. This salty ice is not as strong as frozen pure water. Similarly spreading salt on ice melts it: the salt progressively eats into the ice, forming salty water which is not cold enough to be frozen at the same temperature.
Because ice floats, even large bodies of water that freeze, like some oceans, only form ice on the surface. Most lakes never freeze to the bottom. Even the coldest oceans, say in the Arctic, only freeze on the top, leaving liquid ocean circulating below. Because of this the Earth's oceans are able to redistribute heat and the climate of the earth has less extremes of heat and cold than it would otherwise. If ice were to sink instead of float, the oceans would fill up with ice from the bottom, would remain solid and only some of the top would thaw. A solid ocean would not circulate heat. But because ice floats on the surface the water beneath can continue to circulate and the ice on the surfce stays exposed and readily melts when the temperature rises.
The earth's climate is always changing. When it is very cold it's called an ice age. The most recent ice age finished only ten thousand years ago. During ice ages very large areas of the earth are covered in ice, snow and glaciers. The causes of ice ages are complex, or hard to understand. Global warming is currently affecting the earth's ice resources and its causes are also very complex.
When materials are cooled their molecules vibrate less and compact together. When most materials reach a temperature called the freezing point, the molecules form a crystalline solid - although some materials (like glass and tar) do not crystallise at all but form super stiff fluids, which seem to be solid. Only Helium will not freeze; all other substances will freeze if cold enough, but fluids like cooking oil, anti-freeze, petrol (gasoline), nitrogen, etc. freeze at temperatures that most people will rarely, if ever, experience.
Dry ice.
There is also 'dry ice'; it is frozen carbon dioxide. Dry ice exposed to normal air gives off carbon dioxide gas that is odorless and colorless. The gas is so cold that when it mixes with air it cools the water vapour in the air to fog, which looks like a thick white smoke. It is often used in the theatre to create the appearance of fog or smoke.

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