Can you explain Newton's third law of motion?

I believe the previous answers have been substantial enough to explain the Third Law. I only want to add a point here. Many textbooks try to prove Newton's Third Law from Newton's Second Law. This is not logically sound as Newton's Laws are axioms of Classical Mechanics and one cannot prove axioms from axioms within the same formal system (though it is highly doubtful as to whether Classical Mechanics can be considered such a system). Scrutinize the proofs given in the textbooks and you will find that they have taken an n-particle system and have applied Newton's Second Law and Law of Conservation of Linear Momentum. Note that the latter comes from Newton's Third Law only and hence the proofs are logically incorrect. These type of questions also come in examinations, say, prove Newton's First Law from Newton's Second Law. But school level textbooks of physics usually don't give the complete version of Newton's First Law which runs as follows: "There exist frames of reference with respect to which every body continues to be at rest or in uniform motion in a straight line until compelled by an external force to change that state. Such frames are called inertial frames." Notice that Newton's First Law asserts the existence of Inertial Frames. It is these frames where the Second Law holds. So the first law talks of the inertial frames. What is an inertial frame? Well, many books say that it is a frame where Newton's laws hold. But this somewhat cyclic. If instead, we define inertial frames as those which are at relative rest with respect to a free particle (i.e. a particle with no real forces being acted upon) then the Second Law axiomatically holds in such frames. However, using the Second Law, we cannot actually prove the EXISTENCE of inertial frames as talked of in the First Law. And hence using Newton's Second Law we CANNOT prove his First Law. read less