What is the cause of Brownian motion?

Molecules in a fluid move constantly, freely, randomly, in all directions and with all possible velocities. This erratic motion of the molecules causes them to collide with anything in their path. when pollen grains are suspended in a gas, they fill a resultant force i particular direction and moves to another position, where it has to face same force again. For example, dust particles will be bombarded by the molecules moving at high speeds, causing them to have a zig-zag motion.

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