Newton’s 3rd law:

To every action, there is always an equal and opposite reaction.

If a particle 1 acts on particle 2 with a force \( \vec{F_{12}} \) in the direction along the line joining the particles, simultaneously particle 2 acts on the particle 1 with a force  \( \vec{F_{21}} \), then we can write,  \( \vec{F_{21}}=-\vec{F_{12}} \)

The 3^rd law states that when one body exerts a force on a second body, as a result, the second body simultaneously exerts a force of equal magnitude on the first body. Action and reaction in the statement mean forces and not cause and effect.

When the action and reaction act on different bodies, then the equation of balancing each other does not hold good. Newton’s 3^rd law of motion is true only for the pair-wise interaction. Even there are some systems, where Newton’s 3^rd law of motion is not obeyed by the forces, for example, the electromagnetic forces between moving charged particles. When the forces propagate from one body to another body, the 3^rd law fails for that forces.