You can make a miniature comet and watch as it sublimates--just like a real comet being heated by the Sun! Make sure you have adult supervision. The materials you will need are Dry Ice (solid carbon dioxide), a large bowl, a garbage bag, several smaller plastic bags, gloves, a hammer, water, sand, and a few drops of ammonia. Buy about 3 pounds of Dry Ice. Be very careful in handling Dry Ice, and always wear gloves. Solid carbon dioxide is much colder than ice, and if it touches your skin it will hurt as if you had been burned by fire. Use a plastic garbage bag to line a bowl big enough to hold a quart of water. Put two cups of water into the lined bowl. Add a couple spoonfuls of sand. Sprinkle in a few drops of ammonia and stir the mixture well. Wearing gloves, wrap the dry ice in several plastic bags. Use a hammer to pound the dry ice into small pieces. When the dry ice is crushed add about two cups of it while stirring your comet "soup". Keep stirring while the dry ice freezes the water. When the mixture is almost completely frozen, lift it up using the plastic liner of your bowl and shape the wrapped mixture into a ball. When the "comet" is frozen and can hold its shape on its own, unwrap it and set it somewhere you can watch it. The dry ice will sublimate into a gas. You may see jets of carbon dioxide shoot from your comet. After a while, your comet will shrink and become pitted, like a comet that has been eroded by the Sun. (Based on a Recipe by Dennis Schatz, Pacific Science Center, Seattle, WA.)
Another COMET project developed by UC Berkeley is called "Make a Comet in the Classroom"
Make a Comet in the Classroom