Newton’s third law is among the most widely misunderstood aspects of classical physics—in part because it’s generally described in terms of the archaic phrase “action and reaction.” Actually, the third law describes forces between interacting objects, asserting that the force one object exerts on another is equal and opposite the force that the second object exerts on the first. It does not stand as a separate feature of classical mechanics but rather goes hand-in-hand with the second law to provide a consistent description of motion.