Brownian motion is the seemingly random motion that a small particle (say, a grain of pollen) undergoes when it is suspended in a liquid. First documented by Scottish botanist Robert Brown, it was explained by Einstein as the result of the pollen grain being buffeted by the random motion of molecules in the liquid. Brownian motion is similar to the random walk, and the equations governing Brownian motion can be derived from the random walk equations by making the step size infinitely small along with a few other mathematical assumptions.