I consider myself pretty conversant in metric units. I know that 20 kilograms is 44 pounds (double it and add 10%) and that five kilometers is a little over three miles (I’ve, uh, run a lot of 5Ks).

Every once in a while, you may see a temperature in Celsius instead of Fahrenheit. Your car thermometer may revert to the wrong system and you don’t know how to change it, for example. Since almost ...