Newton's third law

Forces always come in pairs. When one thing pushes or pulls on another, the second pushes or pulls back on the first with a force of the same size in the opposite direction: action and reaction. The two forces act on different things, so they never cancel each other out, and neither can exist without the other. A swimmer pushes the pool wall backwards and the wall pushes the swimmer forwards; a rocket pushes hot gas down and the gas pushes the rocket up; two magnets push each other apart equally. Which one moves more depends on the masses: the lighter one gets the bigger acceleration.