The Kerr metric is the exact solution of Einstein’s field equations for a spinning, uncharged black hole. It defines two critical surfaces: the event horizon (the point of no return for matter and light) and the ergosphere (the stationary-limit surface, where spacetime itself is dragged faster than light). The equatorial slice shows the photon sphere, the innermost stable circular orbit (ISCO), and how the ergosphere swells with increasing spin. Spin is not optional for real astrophysical black holes — accretion torques them up to a ≈ 0.9–0.998. The ergosphere boundary is the lever arm for everything that follows in steps 4 and 5.