Generally, water has been the most widely used substance to control fires. Water typically comes cheaply, is harmless, can be stored at normal temperatures and atmospheric pressure, extinguishes fires, and is typically readily available. The bulk of fires can be put out more effectively with it than with any other known liquid. Depending on the type of structure to be protected, the contents of the building, the intensity of the fire, the predicted fire growth rate, water sensitivity, ambient freezing conditions, and desired time until activation, each system has a specific appropriate usage.