Greetings everyone, I'm new here! I tried searching the forums for my problem, but most deal more with the common misconceptions, not the question why - if there is such a question. Lately, I've been trying to gain an in-depth look into Newton's work, moreover the third law which has been bugging me for quite a while. It states, that whenever an object exerts a force on another object, the second one will also exert a force on the first, equal in magnitude, collinear / opposite in direction. Why? Why does the other object respond with an equal, opposite force? Is there a causality link I'm missing? I'd really like to understand this more. For example, I can understand that when a bird flaps its wings, the wings exert a force on the air, pushing it out of the way, but at the same time the air exerts a force on the wings, giving them an upward force, lift. Perhaps it sounds like a stupid question, but why and how does it happen... When you push on the air, how does it push back on your wings? Is it an intrinsic, observational fact or is there more to it? The more ambiguous case to me is the example of the gravitational interaction between our planet and the Sun. The Sun exerts a gravitational force on Earth and Earth returns the "favor", but considering the mass ratio, the center of rotation is pretty much in the Sun. Why? Why would Earth exert a force on the Sun, equal in magnitude, opposite in direction? Why wouldn't the Sun just push Earth around? Can anybody see what's bothering me? Can you give me a way of looking at this, to find a logical explanation to this... Any insight is helpful! I hope I am making some sense. Thanks everyone! You don't have to give me an answer, just point me in the right direction, books, web resources, anything. I am willing to learn, but sometimes stuff doesn't want to be learned by me.