I know for an action-reaction pair, two forces should act on different objects and equal in magnitude, opposite in direction. Check out this: A brick resting on a flat surface. The normal force acting up is just equal to the weight acting down. Is this an action-reaction pair? My teacher said no. But I think it was an action-reaction pair because brick exerts its weight on the surface,it's a common sense, and because of the weight, the surface exerts normal force on the brick, and they must be equal in magnitude, then how come they are not. My teacher's explanation was like because there were three objects, the other one was earth. The weight came from the earth, it meant the earth exerts brick's weight on the surface? IT doesn't make sense to me. Hope u can help. thanks.