The kilogram is getting a makeover. The metric measurement for mass is the only standard unit still based on a physical object: a metal cylinder kept in France. Now scientists want to redefine the kilogram using a mathematical constant, or number that never changes.

The reason: Over time, the cylinder’s mass has fluctuated. Scientists aren’t sure why. But the subtle shift has made “all other measurements that are based on it inaccurate,” says Ben Stein of the National Institute of Standards and Technology in Maryland.

Scientists have created a sphere of the element silicon (Si) in a very pure form. It weighs the same as the original cylinder. Scientists will calculate the number of atoms—the smallest unit of an element—in it. They can then define a kilogram using the mass of that many silicon atoms. This “ensures people can accurately measure mass from anywhere,” says Stein.