Prince Rupert's Drops.

Internal Tension: Prince Rupert's Drops Prince Rupert's Drops are made by heating annealed glass and dropping it into water to cool it rapidly. This results in an outer layer of tempered glass while the interior remains annealed giving great internal tension. The bulbous part can be hit with a hammer safely, but snapping the long thin tail releases that pressure as it chain reacts down the tail and explosively shatters the drop.