Brownian motion is the random motion of particles suspended in a fluid (a liquid or a gas). It is named after the Scottish botanist Robert Brown, who first described the phenomenon in 1827. Brownian motion is caused by the collision of the fluid molecules with the suspended particles.

In the case of smoke particles, the fluid is air. The smoke particles are very small, so they are easily moved by the air molecules. The collisions of the air molecules with the smoke particles cause the particles to move in a random, zigzagging path.

Brownian motion is important in many physical and biological processes. For example, it is responsible for the spreading of scents through the air and the movement of bacteria in water.