When a pure water sample is cooled it can remain in the liquid state at temperatures well below its melting point (0 °C). The initiation of the transition from the liquid state to ice is called nucleation. Substances that facilitate this transition so that it takes place at a relatively high sub–zero temperature are called ice nucleators. Many living organisms produce ice nucleators. In some cases, plausible reasons for their production have been suggested. In bacteria, they could induce frost damage to their hosts, giving the bacteria access to nutrients. In freeze–tolerant animals, it has been suggested that ice nucleators help to control the ice formation so that it is tolerable to the animal. Such ice nucleators can be called adaptive ice nucleators. There are, however, also examples of ice nucleators in living organisms where the adaptive value is difficult to understand. These ice nucleators might be structures with functions other than facilitating ice formation. These structures might be called incidental ice nucleators.