The document discusses Newton's Third Law of Motion, which states that for every action there is an equal and opposite reaction. It provides several examples to illustrate this law, including rockets propelling forward as hot gases escape out the bottom, cars moving forward as wheels push backwards on the road, a baseball hitting a bat to the left as the bat pushes the ball to the right, and birds staying aloft as their wings push air downwards causing an equal reaction force pushing the birds upwards. It also discusses how fish propel forward as their fins push water backwards.