Isaac Newton's third law of motion states that every action has an equal and opposite reaction.

This means that when an object exerts a force upon another object, that second object responds by exerting the same amount of force upon the first object. Every physical interaction results in two equal forces acting upon each other in opposite directions.

For example, if you were to drop a tennis ball onto a slab of concrete, the ball would exert a downward force on the concrete. At the same time, the concrete would exert the same amount of force in the opposite direction upon the ball. When the ball bounces after hitting the concrete, you are seeing the reactionary force acting upon it.

Not all examples of this law are obvious, yet you are subjected to it at all times. For instance, the chair you may be sitting on is reacting to the force of your weight pushing down on it by pushing back up against you. When you take a step, the same thing happens between you and the ground you walk on. A swimmer propels themselves through water by pushing water behind them, which then exerts the same force in the opposite direction to move them forward.