a Prince Rupert's Drop, a small piece of molten glass is dropped into a
bucket of extremely cold water while the glass is still hot. The
glass forms a characteristic tear-drop shape, with a fat head and a long,
curving tail. The cold water causes the outside of the glass to cool
extremely quickly, while the hot glass inside the head takes longer to
cool. As it cools, it contracts, and creates a state of tension in the
Prince Rupert's Drop.
No matter how hard you try, you cannot break the head of a Prince Rupert's Drop. The glass can be hammered, pinched with pliers, and run through a variety of mechanical trials, but the glass will still hold, because the state of tension is so high. If, however, the tail of the Prince Rupert's Drop is damaged in any way, the entire object will explode, because the tension has been broken. And "explode" is really the right word to use, as a Prince Rupert's Drop will shatter explosively when the tail is damaged.
These interesting demonstrations of the stress of glass were introduced