Flash back for a moment to the fifth grade (if it\u2019s not too painful), when your math teacher asked you to solve a scale puzzle like this one: You have nine coins, identical in appearance. One is counterfeit, and it is heavier than the others. Use the balance scale only twice to figure out which is the heavy one.Now to be perfectly honest you don\u2019t actually need a scale to solve the puzzle. You should dredge one up anyway, though, because it is so elegant and extremely satisfying to use. The balance scale is the simplest form of weighing machine, and the oldest. The always-resourceful Egyptians employed them more than 5,000 years ago, suspending two pans from either end of a beam that hung from a vertical support. They filled one pan with a known weight, the other with the object to be weighed. Before the advent of international standards for weights and measures, civilizations came up with their own. The Greeks and Egyptians used a wheat seed as the smallest unit of weight. The Arabs weighed gold, silver and precious gems using a small bean called the karob (the origin of the word carat).While the balance scale is still in use today, both as machine and metaphor, its descendants have taken over much of its business. Spring scales, such as the one you avoid stepping on in your bathroom, came along in the early 1700s. Today\u2019s electronic scales can weigh tractor-trailers and freight trains, while quartz microbalances can calculate the density of gases weighing up to 0.1 gram.