Newton's third law of motion

Forces always come in pairs: if body A exerts a force on body B, then B exerts an equal and opposite force on A, F_AB = −F_BA.

The two forces of a pair act on different bodies, so they never cancel each other in the equation of motion of a single body.

Action and reaction are simultaneous; neither is the cause of the other, so the labels action and reaction can be swapped.

The pair is always of the same nature (both gravitational, both contact forces, and so on).

The earth pulls a falling stone down and the stone pulls the earth up with an equal force; the earth's acceleration is tiny only because its mass is huge.

Internal forces within a system come in third-law pairs and cancel in the total, so only external forces change the total momentum of a system.