Prepare three identical glass containers of water. Set one container in a pan of water on a heating unit. Leave one in a pan at room temperature. Place the third in a pan of ice cubes.

Hang a tea bag or drop ink or dye in the water in each container. You will observe differences in the diffusion of the tea or dye.

Similarly, perfume can be placed on a handkerchief, and you can time how long it takes before you can smell it. Next, put a different perfume on a different handkerchief. Hold
the handkerchief over a heat source such as radiator or a light bulb, and time the rate at which the odor diffuses.

You will realize that the small particles of the materials move at an increased speed with increased heat.

If you imagine the materials being made up of atomic particles (atoms or molecules), you could picture them bouncing against each other.

When atomic particles hit very hard and are knocked far apart, scientists say that the material is a gas.

When heat is removed, the particles do not hit each other with as much force, thus are closer together, and scientists say the material is a liquid. If more heat is removed, the particles move more slowly and bounce together with even less force. A material in which particles are close together is called a solid.

Scientists have never been able to stop atomic particles from moving (even in ice, they are moving and bumping into each other).

The movement in these ways causes expansion and contraction -- a heated material expands or gets larger, a cooled material contracts or gets smaller.