In general relativity, freely falling bodies follow geodesics. The geodesic deviation equation describes how nearby geodesics converge or diverge due to curvature. For a space of constant sectional curvature K, the separation ξ satisfies D²ξ/dτ² = −K ξ. For positive curvature (K > 0, like a sphere), geodesics that start parallel converge — this is the tidal focusing effect (gravity pulls things together). For negative curvature (K < 0, saddle/hyperbolic), geodesics diverge exponentially — tidal stretching. At K = 0 (flat space), geodesics remain parallel forever. Einstein's equations relate K to the stress-energy tensor, so matter curves geodesics.