Think of the most fussy science teacher you ever had. The one who docked your grade if the sixth decimal place in your answer was rounded incorrectly. Now imagine work that even that teacher would hate for being too anal-retentive. That’s the kind of work that goes into defining the meter, the second, and other international standard units of measure.
Here’s a look at some ridiculously precise standards and the role that elements have played in defining, redefining, and re-redefining them over the ages.