Molecules in a gas move constantly, freely, randomly, in all directions and at high speeds. They are able to do so because the intermolecular force of attraction between the molecules is negligible when in a gaseous state. This constant motion of the molecules causes them to collide with anything in their path. For example, dust particles will be bombarded by the molecules moving at high speeds, causing them to have a zigzag motion.

Brownian motion also takes place in molecules in a liquid, but to a less obvious extent than in a gas.