The Making and Breaking of Prince Rupert's Drop

Prince Rupert's Drop is a string of glass with a bulb on the end formed by pouring molten glass into cold water. The glass immediately sets on the outside of the stream, which insulates the inside of the stream causing it to cool more slowly. As a result the bulb on the end is very hard to break but any stress farther up the strong will cause the whole thing to instantly explode. Here it is in ultra-slo-mo (which is very cool):