State and explain Newton's third law of motion. Write the important points of Newton's third law of motion.

(N/A) Statement: To every action,there is always an equal and opposite reaction. When one object exerts a force on a second object,the second object simultaneously exerts a force equal in magnitude and opposite in direction on the first object.
Important points:
$(1)$ Action and reaction forces always occur in pairs. $A$ single isolated force cannot exist.
$(2)$ Action and reaction forces are equal in magnitude but act in opposite directions.
$(3)$ Action and reaction forces always act on different objects. Therefore,they do not cancel each other out.
$(4)$ Action is the cause and reaction is the effect.
Mathematical representation:
If $\overrightarrow{F}_{AB}$ is the force exerted by object $A$ on object $B$,and $\overrightarrow{F}_{BA}$ is the force exerted by object $B$ on object $A$,then according to Newton's third law:
$\overrightarrow{F}_{AB} = -\overrightarrow{F}_{BA}$
Example: When a spring is compressed by hand,the hand exerts a force on the spring (action),and the spring exerts an equal and opposite restoring force on the hand (reaction).
Note: When considering the motion of a single object,we only account for the force acting on that specific object. If we consider the system of both objects $A$ and $B$,the forces $\overrightarrow{F}_{AB}$ and $\overrightarrow{F}_{BA}$ are internal forces,and their vector sum is zero.