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 fill a balloon with hydrogen, a substance even lighter than helium, the balloon will fly upwards even faster.  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 …