^* Using—how cool is this?—a gravity meter. Walk over an area of very dense rock while holding one of these meters, and you can watch the pull of gravity increase. (Fluctuations in Earth’s density change its gravity enough to pull missiles off their trajectory by as much as a mile or so; gravity maps of Earth were once top-secret Cold War possessions.) This effect is lessened if the dense rock is a tall mountain and you’re four or five miles above the mean surface of Earth. If you carry a bathroom scale to the top of Mt. Everest, you may see that you actually weigh a tiny bit less, not counting the marbles you have obviously lost.