Describe a simple experiment to illustrate that molecules are not at rest, but they constantly move.

The following experiment can be performed to illustrate that molecules are not at rest, but they constantly move :

Take a beaker. Fill it partly with water. Add some lycopodium powder in the beaker containing water. Stir the contents of the beaker with a glass rod. Take out few drops of this suspension on a glass plate. Place the plate on the table and illuminate it with a table lamp. Observe the glass plate through a microscope. It is found that the fine particles of lycopodium powder move rapidly in a random manner and their path is zig zag as shown in figure below.

The reason is that the particles of water are in random motion which collide with the suspended fine particles of lycopodium powder and make them move in a zig zag path.