This document discusses Newton's Third Law of Motion, which states that for every action there is an equal and opposite reaction. It provides two examples: a horse pulling a cart exerts a force on the cart, which exerts an equal force back on the horse, and a space shuttle's rocket boosters propel the shuttle by exerting force on exhaust gases, which exert an equal force back on the boosters. It also answers concept questions about how walking and driving work based on this law.