Prince Rupert’s Drops
Just saw these on hackaday.com. Very cool little things. You can read up on Prince Rupert’s Drops at Wikipedia, but the basic idea is this: Drop molten glass into a cold bucket of water. A tadpole-shaped piece of glass is formed in a way that produces a huge amount of tension within the glass. On the big tadpole end, you can hit it with a hammer, and unlike ordinary glass it won’t break. (Due to the high tension and the egg-like shape distributing the force.) However, if you even scratch the tiny tadpole tail, the entire thing explodes into tiny little shards, almost instantly. (The exploding front propagates down the tail toward the head at ~4200 mph = Mach 5.5)
Even if you didn’t read the above paragraph and/or didn’t care, check out this video of Prince Rupert’s Drops exploding from museumofglass.org. It’s awesome.