Atomic theory says that any liquid is made up of molecules (invisible in 1905). Furthermore, these molecules are always in random, ceaseless motion. The average behavior of these molecules produces the overall properties of any liquid that we observe. But Einstein realized that this random chaos of jostling, invisible molecules would produce statistical fluctuations —for example, once in a while a small group of invisible molecules could, just for move in mostly the same direction. Then, another nearby group of molecules could for a moment move mostly in a different direction. A visible object, immersed among these invisible, randomly jostling molecules, wouldn’t move much most of the time, since it would normally be buffeted from all sides evenly—but then occasionally it could be “pushed” in one direction and then moments later pushed in a different direction, showing a “zigzag” motion.