If you drop molten glass into a bucket of cold water, it cools into a weird tadpole shape: a round head with a long, thin tail. It's called a Prince Rupert's Drop, and no matter how hard you hit the head with a hammer, it won't break. But if you nick the tail even slightly, the whole drop explodes into tiny shards of glass.
That's because the cold water cools the outside of the drop very quickly — so quickly that even by the time the outside is solid, the inside is still molten. As the inside cools, it causes the solid outer layer to contract around it. The resulting stresses actually strengthen the glass — except for the tail, which is too thin to have inner and outer layers to balance the stresses, so it's a weak point.
And if you nick the drop's tail, it doesn't just break, it explodes into tiny pieces, so fast that you can't really see it happen in real time. One instant, the glass is there; the next, it's gone. You can watch it happen at 130,000 frames per second in this video.
What's a Prince Rupert's Drop good for? Researchers have used Prince Rupert's Drops since the 17th century to study material failure and elasticity. Geologists are also interested in these funny glass tadpoles, because similar structures often form during volcanic eruptions.