Brownian motion

Evidence for the movement of particles in liquids came to light in 1827 when Robert Brown, a botanist, observed that fine pollen grains on the surface of the water were not stationary.

 96 years later, in 1923, another scientist called Norbert Wiener explained what Brown had observed. He said that the pollen grains were moving because the much smaller and faster-moving water particles were constantly colliding with them. 

Pollen particle being bombarded by the water molecules

This random motion of visible particles (pollen grains) caused by much smaller, invisible ones (water particles) is known as Brownian motion after the scientist who first observed this phenomenon. 

Brownian motion causes the random motion of the visible particles