I'm long past the days of general physics, but a friend who's taking it now posed an interesting thought experiment, of which to my embarassment, I was unable to answer! He said, imagine a horse is urged to pull a wagon. The horse refuses to try, citing Newton's third law as his defense: "The pull of the horse on the wagon is equal but opposite to the pull of the wagon on the horse" If I can never exert a greater force on the wagon than it exerts on me, how can I ever start the wagon moving? asks the horse. How would you argue with the horse? Stupid, I know...but it's driving me crazy! Anyone remember this stuff???