A version of the Ising Model, an idealization of ferromagnetic materials that involves a square array of spins, each pair of adjacent spins contributes -1 when they are parallel and +1 when they are not. Below a critical value, spins tend to line up, and an overall magnetization spontaneously develops, showing a definite phase transition. (from A New Kind of Science)
S. Wolfram, "Mechanisms in Programs and Nature," A New Kind of Science, Champaign, IL: Wolfram Media, Inc., 2002 pp. 981–983.