Freezing happens when the temperature falls enough for liquid water to turn into ice.

When this happens in nature it is usually because moist air has risen high into the cold atmosphere and cooled. The moisture turns into ice crystals.

Freezing occurs on the ground when the amount of heat being lost by the ground on a winter's night is much greater than the amount of heat being received by the daytime sunshine. This means the ground gets colder and colder and eventually may cool below 0°C, the point at which water freezes.

A freezer is a cabinet in which food can kept frozen for many months.

In the walls of the freezer are pipes which contain a special liquid. It is called a refrigerant. This liquid evaporates inside the pipes and forms a gas. As the liquid evaporates it takes heat from the walls of the freezer. The walls then take heat from the air in the freezer and the air which in turn takes heat from the food.

The refrigerant is squashed from a gas into a liquid by a machine called a compressor. This happens in the back of the freezer. When this change takes place heat is given out into the air outside the freezer. The liquid then goes back to the freezer and allowed to evaporate again and remove more heat.