Prince Rupert's Drop

Prince Rupert's Drop is a piece of glass formed by droping the liquid glass directly into cold water. The resulting piece of glass is highly stressed, so much so that the whole thing eplodes when the delicate tip is broken. At the same time, the larger bulb is extremely strong, able to survive striking by a hammer. Pretty fun physics. Here's a nice video on the subject:

Further details, and the home of the above video is here, and a Wikipedia entry is here.