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Phoenix666 writes:

A Prince Rupert's drop looks like a glass tadpole from a beginner's crafts festival, but it's so strong it can take a hammer hit without breaking. That would be impressive enough, but if you break its tail, which can be done with finger pressure, the drop explodes into powder. The reason for this has mystified scientists for 400 years, but a team from Purdue University, the University of Cambridge, and Tallinn University of Technology in Estonia finally has an answer.

[...]Focusing on the head of the drop instead of the tail, the current study found that the compressive stresses in the glass are about 50 tons per square inch, which gives it the strength of some steels. According to the team, this is because the outside of the drop cools faster than the inside. This turns the outside into a layer of powerful compressive forces pushing inward. These are balanced out by the tensile or pulling forces inside the drop.

So long as these forces remain in balance, the drop remains stable and can withstand tremendous punishment. Normally, because glass is a supercooled liquid rather than a solid, any cracks in the surface propagate at the speed of sound through a glass object, breaking it.

But in a Prince Rupert's drop, the interface between the inner and outer regions deflects the forces sideways, so the crack can't propagate. However, if the tail is broken, The shallow cracks in the tail shoot parallel to the axis of the drop, deep into the head, and into the interface. The damage is so great that the balanced forces are released, causing the drop to explode.

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