Brownian motion is the random movement of small particles suspended in a fluid or gas, caused by collisions with molecules of the fluid or gas. When smoke particles are suspended in air, they exhibit Brownian motion. The motion of the smoke particles is caused by the random collisions of air molecules with the particles. This motion is evidence that air is made of small particles, because if air was continuous and didn't contain any particles, then the smoke particles wouldn't be able to move in a random fashion.