When Einstein was working toward his PhD, he was among the first to explain how particles exhibit random motions in fluids. Diffusion is an important physical process and the Stokes–Einstein relationship describes how particles diffuse through a fluid based on the hydrodynamic theory. But mysteriously at low temperatures below the melting temperature, something changes in supercooled liquids and the resulting highly viscous glassy behavior can no longer be explained by the simple Stokes–Einstein relationship.