Materials scientists have been studying the Prince Rupert's Drop (PRD) since the 17th century at least, as the Royal Society in London has papers detailing its properties from that time. A PRD is a little piece of glass that looks like a curved water droplet or a tadpole with a long tail.
You make them by letting molten glass drip into a bucket of very cold water. The outside of the droplet hardens first, and as the still-molten glass in the middle of the drop cools, it contracts against the inside of the already-hardened shell and forms a type of toughened glass. If you hit the bulb of the droplet with a hammer, it won't break, but snapping the thin tail will cause the entire structure to shatter due to the stress from the contracted glass inside the drop.
But what if you shoot one?
Destin Sandlin of Smarter Every Day did just that with a .22 rifle in the video above, and he filmed it a 150,000 frames per second with a high-speed camera. The resulting footage is captivating.
The bullet striking the bulb of the PRD, like a hammer blow, does not break it. But, a shockwave sent down the structure of the PRD causes the tip of the tail to break off, at which point the entire thing shatters. That said, when Destin shoots a big enough PRD, it actually doesn't break at all. The bullet strikes the large glass bulb and shatters into fragments of lead, but the glass teardrop remains completely intact.
If you want to learn more about PRDs you can see the Smarter Every Day about making them here. And if you are going to try this at home, make sure you've got your safety goggles on.