In this video, Popular Science goes to the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland, to find out just exactly how and why the mole changed. Maybe you’ve heard of NIST—they do stuff like keep the official time for our country with their atomic clock. Which, when you think about it, is a pretty awesome thing. Basically, NIST is our country’s check and balance on, well, everything. From time to the dosage of a medication to what a nickel is made of. They also have the 130-year-old national prototype of the kilogram, known as K20. That kilogram and the international prototype it’s based on in France, Le Grand K, is central to the making of the new mole. Well, that and one of the most perfectly round objects in the world, a one kilogram sphere of silicon. And counting the atoms inside it is where our story starts.