Newton's Third Law states "For any action, there is an equal and
opposite reaction." By "action" Newton meant a force, so for every force
one body exerts on another body, that second body exerts a force of
equal magnitude but opposite direction back on the first body. Since all
forces are exerted by bodies (either directly or indirectly), all forces
come in pairs, one acting on each of the bodies interacting.
Though there may be two equal and opposite forces acting on a single
body, it is important to remember that each of the forces a Third Law
pair acts on a separate body. This can sometimes be confusing when there
are multiple Third Law pairs at work. Below are some examples of
situations where multiple Third Law pairs occur.