Brownian motion is the random movement of tiny particles suspended in a fluid, such as a gas or liquid. This phenomenon occurs because the particles are constantly colliding with the much smaller molecules of the fluid, causing them to move in unpredictable directions. The concept was first observed by the botanist Robert Brown in 1827 when he noticed pollen grains moving erratically in water. This motion is a key evidence for the existence of atoms and molecules, supporting the kinetic theory of matter, which explains how temperature and energy affect particle movement.