Why do teardrops explode?

Prince Rupert's teardrops, that is.

First, some background from New Scientist:

In 1661, King Charles II asked his personal scientific society, later to become the Royal Society, why do teardrops explode? The teardrops he had in mind were the size of a fingernail and made of glass, and they had been brought to England by Prince Rupert of Bavaria. Prince Rupert's drops, as they are now known, had a quite remarkable property: they were immensely strong, able to withstand the weight of a 1-tonne wagon without shattering, but at the touch of a finger they exploded like a bomb.

The video at the top shows the creation of a Prince Rupert drop, but the explosion is done underwater; the second video demonstrates the explosion more impressively by doing it inside a glass jar. When the tail is cracked, the crack crack "races up the tail towards the head at almost 7000 kilometres an hour - three times the speed of Concorde." The explosion occurs in a few millionths of a second, leaving only glass powder. For details about the physics involved, see the New Scientist article at the link.

Thanks to Metafilter for posting the links this morning.