What's heavier, an ounce of feathers or an ounce of gold?
It's kind of a trick question that has to do with the curious and intricate non-metric system of weights and measures that used to be employed:
Gold, as I understand it, is traditionally measured in troy ounces (twelve ounces to the troy pound, which consists of 5760 grains; each grain was originally "the weight of a barley corn taken from the middle of the ear"), along with other fine metals. Each troy ounce is about 31.1 g.
Most other materials are measured in ounces avoirdupois (if you use ounces)—sixteen ounces to the pound avoirdupois, which consists of 7000 grains. Each ounce avoirdupois is about 28.3 g.
Therefore, an ounce of gold weighs more than an ounce of feathers. QED. (But a pound of gold weighs less [ca. 373 g] than a pound of feathers [ca. 453 g].)
(There were also apothecaries' weights, but the apothecaries' pound and ounce are the same as the troy pound and ounce. They differ in the subdivisions, as an apothecaries' ounce is divided into eight drams of three scruples each [1 ap. dram ~ 3.9 g; 1 scruple ~ 1.3 g], while a troy ounce is divided into 20 pennyweights [abbreviation: dwt; 1 dwt ~ 1.6 g] and an ounce avoirdupois into 16 drams [1 dram avdp ~ 1.8 g]. All makes eminent sense, doesn't it? *snerk*)