All the scales in the world—from those at your local deli to the big ones at the junkyard—are ultimately calibrated against a 125-year-old chunk of metal in a vault on the outskirts of Paris. Its mass is the definition of a kilogram. There’s just one problem: That chunk of metal seems to be losing weight. We talk to Derek Muller, creator of the Veritasium science education series, about the race to redefine the kilo in less slippery terms—and what a near-perfect silicon crystal sphere has to do with it. And Jon Pratt, an engineer at the National Institute of Standards and Technology, tells us about his team’s alternative “watt balance” strategy to lock down the kilo in terms of Planck’s constant.