The observation that a mixture of gas molecules of different masses does not behave as expected when subjected to gravity was systematically described by Thomas Graham early in the nineteenth century. Rather than sorting according to their masses, Graham observed that a true mixture was achieved. It was clear that a force must exist to oppose gravity and facilitate the intermingling of gases, but what could be the nature of that force? Even earlier, in 1827, Robert Brown had used a microscope to observe that pollen grains suspended in water exhibit random patterns of motion. Brown reasoned that these motions must be caused by randomly arranged forces in the molecular realm, but once again the nature of such forces was not apparent. It was not until several decades later that Albert Einstein, working on issues concerned with thermal and kinetic energy, provided a theoretical explanation. Einstein reasoned that the thermal energy contained within microscopic bodies is transformed into kinetic energy providing a means for velocity. In the case of Graham’s gases we can use Einstein’s theory to explain how suspended molecules collide with one another in “random walks,” providing the potential for an upward force vector that opposes gravity and sustains a random mixture. In the case of Brown’s pollen grains we can use the theory to explain the perpetual motion of water molecules with random collisions occurring between molecules and grains, forcing the grains to vibrate and move through the water. We now understand that Einstein’s theories on energy and motion explain one of the fundamental processes by which mass is transported at the microscopic scale – molecular diffusion.