A random movement of pollen grains suspended in water was observed by botanist Robert Brown in 1827. He thought that the movement must be due to the live pollen, but on observing non-living dye particles in water, he saw the same random dance. This motion was explained a generation later by Maxwell and Einstein. It's due to the invisible water molecules hitting the visible particles and moving them a bit. Since the particles are hit continuously from all sides, they moves in a random fashion.
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