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. For example, when you push against a wall, your hand applies a force to the wall, and the wall applies an equal force back on your hand. This principle explains various phenomena, such as how rockets propel themselves by expelling gas in one direction while moving in the opposite direction.