What law states "for every action there is an equal and opposite reaction?"
Newton's third law of motion states that forces always occur in pairs of equal magnitude but opposite direction.
If an object
For example, when you are sitting on a chair, the earth is applying a force of gravity on you. You will also be applying a force of gravity equal in magnitude but opposite in direction on the earth. This is one pair of force.
While the earth is pulling you down, the chair will be pushing you up. Even though these two forces are equal in magnitude and opposite in direction, this is not a pair of forces because three objects (the earth, the chair, and you) are involved. The force pair to the chair pushing you up is the force you are exerting on the chair.