Any agent that promotes the formation of ice crystals. Such agents include minute solid particles, such as dust or food particles, and large molecules. Certain organisms living in cold climates, such as some insects, molluscs, and nematodes, may manufacture ice-nucleating proteins in order to achieve a slow controlled freezing of their extracellular body water when the temperature falls below freezing. The structure of these proteins encourages the binding of water molecules and prevents sudden and damaging ice formation. However, gut particles can also act as ice-nucleating agents, and hence organisms that cannot tolerate freezing must void their guts at the onset of freezing conditions. See also cryoprotectant.