Unless a method of preservation is devised, fresh fish quickly degrades. A technique for preserving food called drying involves eliminating the water from the food, which prevents the growth of bacteria. Food preservation techniques dating back to prehistoric times include open air drying using the sun and wind. Normally, water is removed from food via evaporation (air drying, sun drying, smoking, or wind drying), but when food is freeze-dried, the water is first sublimated out of the food through freezing. Food that has been dried successfully kills bacteria, yeasts, and moulds since they require water to flourish.