H088
Which weighs more: a pound of feathers or a pound of gold?
It's the classic trick question, and the classic answer is "they weigh the same." But gold isn't weighed in the same pounds as feathers.
Same word, different weights
Precious metals are weighed in troy units; almost everything else in avoirdupois
The same unit name can mean different amounts depending on what is being measured. Feathers are weighed in ordinary avoirdupois pounds of 16 ounces, about 454 grams. Gold is weighed in troy pounds of only 12 ounces, about 373 grams. So a pound of feathers is heavier. But each troy ounce is bigger than an ordinary ounce, so an ounce of gold outweighs an ounce of feathers.
When two numbers share a unit name, check that they share a definition. "Pound," "ton," "cup," "gallon," and "mile" all have more than one official version.
Try it somewhere else
A recipe from Britain calls for a pint of milk, and you measure out a U.S. pint.
Will you have the amount the recipe expects?
Other words that hide two sizes
A British (imperial) pint is 20 fluid ounces; a U.S. pint is 16, so you would be about 20% short. A "ton" can be a short ton (2,000 lb, U.S.), a long ton (2,240 lb, U.K.), or a metric tonne (about 2,205 lb). A nautical mile is about 6,076 feet versus 5,280 for a land mile. Gold prices today are still quoted per troy ounce.
Related exhibits
Sources: Troy and avoirdupois units: National Institute of Standards and Technology (NIST), Handbook 44, Appendix C. Pint and ton definitions: NIST; UK Weights and Measures Act 1985.