Fill a container with water, and place a drop of ink or dye on the surface. You will see that the particles gradually spread through the whole container, even though they have not been stirred.

Now, close the doors and windows in the room. Place several drops of perfume on a handkerchief, and place the handkerchief near one corner of the room. Quickly go to the opposite corner of the room, and sit down.

Sit as still as possible so that there is very little wind motion in the room.

Count slowly to yourself, and see how far you have to count before you are sure you can smell the perfume.

Compare this demonstration with the previous one, in which ink or dye was placed on the surface of a container of water. (In both cases, the particles of the material gradually diffused, seemingly of their own accord).

Why do you think that the color and the fragrance diffused?

Scientists believe that the spreading is caused by particles bouncing against each other in a random fashion -- much like marbles bouncing together when they are shaken in a box.

This movement is called the Brownian movement.

You might be interested in learning more about the work of Robert Brown (1773-1858), the Scottish botanist whose observations provided evidence that small particles are always in motion.