To the casual observer, Prince Rupert’s drops look like miniature glass sculptures of tadpoles, the kind one might purchase at Pier 1 for living room art. But they’re one of humanity’s most fascinating creations, containing vast secrets.

They can withstand blows from a hammer, the punishing crush of a hydraulic press and even a bullet speeding from the barrel of a .22-caliber handgun. Their tails, however, which narrow into thin threads, serve as a surprising Achilles’ heel. Snip the tails with a pair of pliers, or simply snap them between your fingers, and with an audible pop, the entire drop shatters into a fine powder, leaving no trace of what existed microseconds earlier.

We’ve long known the drops are strong, and we’ve long known why. But researchers at a team from Purdue University, the University of Cambridge and Tallinn University of Technology have quantified just how strong, recently publishing their results in the journal Applied Physics Letters.

The compressive forces in the drop’s fat head begin at 29 tons per square inch and reach up to reach 50 tons per square inch, they said. That’s as strong as some grades of steel.

(Purdue University’s news release for the study is titled “Research solves centuries-old riddle of Prince Rupert’s drops,” but this headline is slightly misleading. The study quantified the glass’s compressive strength).

For all their mystery, Prince Rupert’s drops are fairly simple to create — anyone can do it. Just melt some sort of glass with a high thermal expansion coefficient (i.e., glass that expands upon heating), such as soda-lime glass (the kind used in most bottles, jars and windowpanes), and let a molten drop fall into cold water.

The water immediately cools the outside of the melted glass into a solid, before cooling the inside. This results in strong compressive (or pushing) forces on the outside and strong tensile (or pulling) forces on the inside. The resulting tension, as Smithsonian noted, makes the glass strong. It also means the entire structure is not in equilibrium.

If the tail is broken, the pent-up energy from that tension is suddenly released and cracks shoot through the entire structure at 4,000 miles per hour, which is why the drop then shatters into such a fine dust.

No one is sure when the glass teardrops were first made and observed. They gained their name in the 17th century, when Prince Rupert of Bavaria gave five of them to King Charles II of England. They were then passed onto the Royal Society in 1661, New Atlas reported.

In the centuries that followed, though, exactly how strong they were remained a mystery, until Tallinn University scientist Hill Aben created a technique called integrated photoelasticity. Employing this, researchers suspend a three-dimensional transparent object in an immersion bath, then beam polarized light through it. When the light hits the object, the entire thing lights up a beautiful rainbow of colors, which highlight the object’s stress distribution.

By focusing on the incredibly strong head of the drop, the researchers were able to finally measure the compressive stresses of the glass.

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