Some smoke is blown into a small glass pot that contains air and is then viewed through a microscope. The air molecules collide with much larger smoke particles and if a strong light is shone on the box the smoke particles can be seen They judder around as they are hit from all sides by the much smaller and invisible air molecules. The smoke particles can be seen because light is scattered from them and they look like tiny stars.

This motion is known as Brownian motion.

A similar effect can be seen by viewing the motion of pollen grains in a liquid.