Place a drop of dark food coloring or black ink in a clear glass of cold water. At the same time, place another drop into a glass of hot, but not steaming, water. Both the coloring and the water are composed of tiny particles called molecules.

Note how long it takes the coloring to disperse in each container. Hypothesize about why the coloring seems to move about more quickly (without stirring) in the hotter water.

The more rapidly moving molecules of the hot water bounce against the molecules of the coloring and scatter or diffuse them faster.

Now repeat this activity using water at various temperatures, and speculate about how fast the molecules of coloring will move in each case.

Next, predict what would happen if you put equal amounts of instant coffee or cocoa in identical cups of cold water and hot water? Check your predictions.

The material will mix faster in hot water.

Do you think odors spread faster on hot or on cold days?

When the movement of the molecules in a material increases (as it does with heat), the molecules bump into each other with more force and knock each other further apart. This can be demonstrated by having marbles represent molecules.

Shoot the marbles into other marbles.

You will realize how this model can explain the expansion and contraction of materials.