The 3D Ising model on a cubic lattice: spins s=±1 interact with neighbors and external field H. Energy = −J Σ s_i s_j − H Σ s_i. Metropolis Monte Carlo samples the Boltzmann distribution. Critical temperature T_c ≈ 4.51 J/k_B for simple cubic; near T_c, correlation length diverges and domains of all sizes appear. Red = spin up, blue = spin down.