The Third of Newton is hard to wrap your mind around because many of the examples you think are describing the law are really just describing ordinary pushing. It's convenient to try to think of a rocket as pushing against something by means of its exhaust. But a better way of thinking about it (informally, anyways) is the rocket pushing against its exhaust. Now that's really not physically accurate, but helps if you think of the thrust chamber as trying to separate -- or increase the distance between -- the rocket and its exhaust.
If you stand between two shopping carts (again, well oiled), one filled with soup cans and the other filled with bathroom tissue, and you push them apart, the one with the cans doesn't go as easily as the one with bathroom tissue. That gives you a sort of practical notion of the relationship between force, mass, and acceleration.
Now think of Sts60's experiment again. Sit in your office chair (well oiled) on smooth hard flooring and toss a bag of rocks, or some books. You'll move. But why? Is it because the thing you threw pushed against the air? No, it's because you pushed against the books. The books responded by going away from you fast; but you responded also by moving away from the books -- albeit slower because you have more mass.
That's why we talk of conservation of momentum. The combination of velocity and mass is momentum, and it's that number that is equal in both directions. The momentum of the books one way is equal to the momentum of you the other way. Momentum is said to be "conserved" because the velocities that make up the momentum are in opposite directions. We consider them positive and negative numbers respectively, so the net sum is zero -- conservation.
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