The typical elementary-school explanation of the difference between mass and weight goes something like the following: Mass is the amount of matter contained in an object. If you travel to the Moon, another planet, or anywhere far away from Earth, your mass doesn't change. Weight is how hard Earth pulls on you. When you travel to the Moon or another planet, the pull of gravity changes, and so your weight changes. Although this explanation is correct, the author never found this definition to be satisfactory. Here he provides a much more complete explanation of the difference between mass and weight by explaining one of Newton's famous laws of motion. (Contains 2 figures.)