The metric unit of mass is the kilogram and just like all your favorite superheroes, it's undergoing a reboot. This has been a path for most standard units of measure over the last century. There used to be a "physical" standard for the meter, meaning one that you could potentially hold. But this has been replaced with an astronomical standard based upon the average distance from the Earth to the Sun. The replacement of standard unit of time, second, has been replaced the fastest from special clocks to the vibrations of Cs atoms. As measurement techniques improve, it seems inevitable that all standards for measurement should be based upon aspects of nature that appear unchanging (at least on a human timescale).