Water is the commonest substance which undergoes a change of state within the temperature range where human life is possible. By a change of state is meant a change between solid and liquid, or liquid and gas, more rarely a change between solid and gas, skipping the liquid state. Most pure substances change very abruptly from one state to another. The freezing point however does not depend only on temperature, but on pressure. Frost also forms peculiar rings of stones on the surface. Apparently heat leaks out quicker through a solid stone than through soil, so ice forms under stones and lifts them to the surface. In the Arctic there are no earthworms to counteract this process. In England frost is not much more than a nuisance. In the northern parts of the Soviet Union it is one of the main natural conditions to be fought.