Newton's third law of motion states that for every action, there is an equal and opposite reaction. The document provides examples of this law, such as bumper cars pushing against each other with equal forces in opposite directions. It also explains that it can be difficult to identify the action-reaction pair when one object is much more massive than the other and does not noticeably move, such as the Earth when a person walks on it. The document asks the reader to think of additional examples of Newton's third law of motion.