A pound of feathers has the same weight as a pound of gold, but they don't have the same mass. Which has more, and why?
Gold (19.3 grams per cubic centimeter) is much denser than the keratin that makes up feathers (1.3g/cc). That means that 1 pound of gold displaces about 23.5 cc of air, while 1 pound of feathers displaces about 348 cc of air, not counting any air inside the feather shaft. The displaced air is pushing up on them with a force equal that the weight of that volume of air, so it takes a slightly greater mass of feathers to weigh a pound. Of course, if you weighed them both in a vacuum, their mass would be the same.