When a small amount of molten glass is dropped into cold water the result is a Rupert's drop (also called a Prince Rupert's drop). The Rupert's drop gets its name from King Charles II's nephew Rupert who brought the novelty from Germany to England in the 1600's.
At first glance, Rupert's drops are unspectacular to look at -- tear shaped pieces of glass that trail off to thread-like tails. A Rupert's drop, though, is a fascinating example of the physics of glass and an ideal illustration of the importance of proper annealing.
When the drop of molten glass hits the cold water, the out layer of glass quickly cools, contracts and hardens. That puts the still-molten glass on the inside under tremendous pressure. The result is that the bulb-end of the drop is highly tempered -- often able to withstand a full hammer blow without breaking!
The glass is stressed throughout though, including the thin and fragile tail. Break that tail -- or even scratch the glass's surface and the glass will explode violently into fine glass frit.
