video Make Prince Rupert's Drops - Glass Turned Hand Grenade
Prince Rupert's Drops are one of the most spectacular physics demonstrations I have witnessed. They are formed by a drop of molten glass being cooled rapidly in water in such a way that stress in the core is at a tipping point that requires only a slight imbalance to explode. Even so, the stresses are organized pressing against each other so that if the drops of glass are impacted from the side they can withstand direct hammer blows. It is not until the tail of the drop is fractured that the forces within the glass lose their balance and release in an explosion, fracturing through the glass at a rate three times faster than a .50 BMG sniper rifle bullet.
At 3600 meters per second, the rate that Prince Rupert's Drops fracture surpasses the rate of any other release of kinetic energy that I am aware of. Reactions involving chemical and nuclear energy are typically the only events that are capable of accelerating to this this extreme speed.
The drops are quite simple to make as the above video demonstrates, so long as great care is placed into safety regarding eye protection and care to avoid burns. The glass from these drops can be thrown great distances, so they should be broken within confinement to avoid throwing shards all over the work area.