A phenomenon in which the constituents of a system cannot be regarded as acting independently from each other. Cooperative phenomena result from interactions between the constituents. Phenomena that can be described by the liquid-drop model of nuclei, such as nuclear fission, are examples of cooperative phenomena because they involve the nucleus as a whole rather than individual nucleons. Other examples of cooperative phenomena occur when a substance undergoes a phase transition, as in the phenomena of ferromagnetism (see magnetism) or superconductivity.