Brownian motion is the continuous, random movement of microscopic particles suspended in a fluid (liquid or gas), caused by their incessant bombardment by the much smaller, rapidly moving molecules of the surrounding medium. The motion is not driven by any intrinsic propulsion of the particles themselves, rather, it arises from the unequal and constantly fluctuating impacts of fluid molecules, which, due to thermal energy, are in perpetual motion. Because these molecular impacts are highly irregular in magnitude and direction, the resulting trajectory of a suspended particle is erratic and unpredictable.
