Brownian motion is the random movement of particles suspended in a fluid (a liquid or a gas).
The discovery of Brownian motion served as important evidence for the kinetic model of matter:
Observation: When viewed through a microscope, particles of smoke or pollen in air do not stand still but instead are in constant motion in jerky randomly changing directions.
Explanation: The air must be made up of many tiny particles which are colliding with the smoke particles. These collisions cause the random motion of the dust particle.