Cracks in the Sidewalk

You will need:
 * Empty can
 * Hammer
 * Nail
 * 100-watt light bulb
 * Aluminum foil
 * Pliers or tongs
 * Adult supervision

Hammer a nail into the bottom of the empty can. Make sure the nail can glide in and out of the can smoothly. Put a foil shade over the top half of the lightbulb. Using pliers to hold the nail, heat the nail over the lightbulb for two minutes. Try putting the nail back in the hole.

You will notice the nail does not fit. This is because the heat makes the molecules expand, making the nail larger than it was before. The same thing happens to the molecules in the sidewalk. If there were no cracks in the sidewalk, it would buckle in the heat, as occasionally occurs with asphalt.

Variations:

Fill a 2-liter pop bottle about half-way with hot water and swish it around. quickly put the lid on the bottle and tighten it. Observe the pop bottle. It will eventually collapse as the air molecules get colder and shrink. This is the opposite of the above experiment. Put the pop bottle under hot water and watch it expand. This is similar to the above experiment.