The kilogram is the only unit of measurement still defined by a physical object. "Le Grand K" spends most of its life in a heavily guarded, subterranean vault on the outskirts of Paris. But what happens if it gets heavier from dirt or lighter through cleaning? What happens to all other kilogram standards across the world? Sue Nelson talks to Ian Robinson from the UK's National Physical Laboratory, who is trying to redefine the kilogram and tie its measurement to the fundamental constants of nature.