Spin ice: Ising spins on each bond of a square ice lattice, constrained by the "2-in, 2-out" ice rule at every vertex (analogous to Bernal-Fowler water ice rules). Violations create pairs of magnetic monopoles connected by a Dirac string of flipped spins. At low T, ice-rule states dominate — the ground state is massively degenerate (Pauling entropy). Increasing T excites monopole pairs.