In 1868 the German physician Carl Wunderlich published the normal temperature of a healthy person as 37 degrees Celsius, from readings he said ran past a million, taken under armpits in Leipzig. A round number, offered as a round number. Converted to Fahrenheit, 37 comes out as exactly 98.6, and in that form it looks as though somebody measured it to the tenth of a degree. Nobody did. The decimal came from the arithmetic, not from anyone’s arm. A Stanford study in 2020 suggested that people in the United States now run a little cooler than that anyway, by something like a degree Fahrenheit since the Civil War. So the most precise-looking number in the medicine cabinet was never precise, and may not even be ours any more.