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Brownian motion is the random motion of particles in a liquid or a gas. The motion is caused by fast-moving atoms or molecules that hit the particles. The Brownian Motion was discovered in 1827 by the botanist Robert Brown. In 1827, Brown noticed in a microscope that particles were trapped in the tiny holes of pollen grains in water. Brown saw the particles moving in the water, but he was not able to find out how. Scientists had thought for a long time that atoms and molecules were the main parts of matter. Albert Einstein published a paper in 1905 that said the motion of the pollen was caused by individual water molecules. Jean Perrin verified the idea in 1908. Perrin discovered the movement of the particles was because they were being hit by atoms at random. Perrin won the Nobel Peace Prize for his work.
The random movements can't be understood by scientific models. Scientists can only guess what might happen to particles that small.