I've always been fascinated by Prince Rupert's Drops. If you're not familiar with them, know that Prince Rupert's Drops are hardened droplets of glass; because of the nature of the tension that formed when they hardened, they are at once durable and delicate. The heads of Prince Rupert's Drops are very strong: one can strike them with hard objects or against hard objects and they'll be all right. In contrast, their tails are very fragile: one can snap them between one's fingers.
Most interestingly, breaking the tails of Prince Rupert's Drops causes the droplets themselves to instantly turn to dust. This feature has not been lost on those who craft metaphors, and rightly so. Aren't our very hearts like Prince Rupert's Drops?
But why read when you can watch? THIS IS 2008, DAMN IT!*
Prince Rupert's Drop
Exploding Glass: Rupert's Drop - Museum of Glass
* Unless you adhere to, say, the Ethiopian calendar. If you do adhere to the Ethiopian calendar, incidentally, Happy New Year!