Mean-field Ising: replace all neighbors with average field → self-consistency equation m = tanh(βJzm + βh), where β=1/T, z = coordination number. Phase transition at T_c = Jz/k_B (β_c·Jz=1): for T < T_c and h=0, spontaneous magnetization m≠0 exists (two solutions ±m*). Symmetry breaking: choosing ±m* = choosing a ground state. The graphical intersection of y=tanh(βJzm) and y=m reveals stable/unstable fixed points. In 3D: T_c ≈ 4.5 K for iron (mean field overestimates by ~2×).