In the late 17th century CE, Prince Rupert’s drops were some of the most confusing objects known to science: an extremely tough glass teardrop that will disintegrate if its tail is even slightly damaged.
Prince Rupert (of the Rhine) didn’t discover them, but he did popularise them in England, hence the name. The drops are easy enough to make: get some molten glass and drop it into cold water. A teardrop shape is formed as the outside of the glass cools and solidifies.
This is the weird bit: the head of the teardrop is tough. Like, really tough. You can hit it with a hammer and it won’t break. You can put it in a hydraulic press and it will sustain tonnes of weight, even denting the steel end of the press. But if the tail end gets even a tiny bit of damage, boom. The entire drop disintegrates, just disappears instantly leaving only a fine powder. You can see a demonstration of them in action in the video below.
Why do they do this? Two reasons: compression and tension. The large end of a Prince Rupert’s drop is highly compressed, much like tempered glass / safety glass. (Incidentally, we only have tempered glass because of the drops – Francois Barthelemy Alfred Royer de la Bastie was probably inspired by the weird properties of Prince Rupert’s drops when he invented it in 1874.) Because the drop cools from the outside in, the interior of the drop has very high tension, held in check by the compressed exterior. As soon as that exterior is damaged (say by snipping off the tail) the tension is released all at once and the entire thing disappears into dust.
One more interesting thing: there are also naturally occurring Prince Rupert’s drops, formed by lava dropped into water. They’re called Pele’s tears, after the Hawai’ian goddess of fire and volcanoes.