Introduce students to amorphous substances.
Heat glass to the point gravity pulls it down off of a solid glass rod forming a droplet of solid glass. Have the droplet fall into a large beaker filled with water. The outside contracts violently while the inside remains hot and expanded. The result causes great internal stress which focuses at the tail. The fat part part is stronger than steel and my be smashed with a flat rock very hard and survive, but just bend the tail a little and it shatters into hundreds of tiny pieces. A lot of energy is stored and finally released, so Prince Rupert Drops are dangerous. Never let students take any out of the room and never leave any whole, always twist that tail!