θ̈ + γθ̇ + sin θ = A cos(Ωt). The undriven pendulum has a separatrix in phase space separating oscillation from rotation, passing through the unstable equilibrium (π, 0). With weak drive and damping, limit cycles appear. Increase the drive amplitude A past a critical value: chaotic tumbling emerges as the pendulum intermittently rotates or oscillates. The Poincaré section (sampled once per drive period) reveals the strange attractor.