If you fill a balloon with helium, a substance lighter than the nitrogen, oxygen and other elements which compose the air around it, the balloon will immediately fly upwards. If you blow a dandelion seed out of your hands, a substance just barely heavier than the air, it will float away and slowly but eventually fall to the ground. And if you drop an anvil from your hands, something much heavier than the air, it will quickly and directly fall straight to the ground. Now, this has...