The Third Law of Motion, formulated by Isaac Newton, states that for every action, there is an equal and opposite reaction. This means that when one object exerts a force on another, the second object exerts a force of the same magnitude but in the opposite direction on the first object. This principle helps explain various physical phenomena, from walking to rocket propulsion. In practical terms, when you push against a wall, the wall pushes back with equal force. This law is fundamental in understanding how forces interact in our everyday lives and is a cornerstone of classical mechanics.