You’ve probably seen the videos. A heavy hammer swings down with enough force to crush bone, yet the tiny glass tadpole underneath doesn't even flinch. It’s a Prince Rupert Drop. Honestly, it looks like a glitch in the simulation.
These things are weird.
They’re made by dripping molten glass into cold water. That’s it. No special chemicals or futuristic polymers. Just heat and a bucket of water. But the result is a piece of glass where the head is essentially as strong as steel, while the tail is so fragile that a flick of your fingernail causes the whole thing to vanish into a cloud of dust.
The Physics of Why a Prince Rupert Drop Breaks Like That
Most people think glass is just glass. Brittle. Fragile. But Prince Rupert drop breaking mechanics are a masterclass in internal stress. When that molten blob hits the water, the outside cools almost instantly. It forms a hard shell.
But the inside? It’s still liquid.
As that inner core eventually cools, it tries to shrink. But it’s stuck to the outer shell. It pulls on the outside from the inside, creating massive compressive stress on the surface and tensile stress in the center. We’re talking about 700 megapascals (roughly 100,000 psi) of pressure.
That’s why you can shoot the head with a .22 caliber bullet—like Destin Sandlin did on SmarterEveryDay—and the bullet literally shatters against the glass. The glass doesn't break because the surface is being squeezed so tight that cracks physically can't find a way to start.
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Why the Tail is the Kill Switch
If the head is a tank, the tail is the self-destruct button.
The tail is thin. It doesn't have that thick, protective layer of compression. When you snip just the tip of that tail, you’re releasing all that stored energy at once. It’s like a spring that’s been wound too tight for four centuries.
Once a crack starts in the tail, it enters the "tension zone" in the middle of the drop. From there, it’s a race. The fracture propagates toward the head at speeds between 1,450 to 1,900 meters per second.
That is over 4,000 miles per hour.
It’s faster than the speed of sound in air. You can't even see it with a regular camera. You need equipment running at hundreds of thousands of frames per second just to catch a blurry streak of the disintegration.
The 2017 Breakthrough That Solved the Mystery
For a long time, we didn't actually know exactly how much pressure was inside. We just knew it was "a lot." In 2017, researchers from Purdue University and the University of Cambridge finally mapped the stress.
They used a transmission polariscope.
Basically, they shone red LED light through the glass and measured how the light slowed down (birefringence). This gave them a rainbow-colored map of the forces. They found that the super-strong outer layer is actually quite thin—only about 10% of the drop's total diameter.
If a crack manages to penetrate that 10%, the drop is toast.
Real-World Tech Inspired by Glass Tears
This isn't just a cool party trick or a way to blow up glass in slow motion. The principle behind the Prince Rupert Drop—thermal tempering—is the reason your smartphone screen doesn't shatter when you breathe on it.
- Gorilla Glass: Uses chemical tempering to create a similar "compression skin."
- Car Windows: Designed to shatter into tiny, blunt cubes rather than jagged shards using these exact stress principles.
- Volcanology: Scientists at the University of Bristol study these drops to understand how magma fragments during eruptions.
How to Experience This Safely (If You’re Brave)
If you want to witness Prince Rupert drop breaking in person, you can actually buy them or make them if you have a torch. But be careful. These aren't normal glass breaks.
- Always wear eye protection. The glass doesn't just "break"; it explodes. The energy release turns the drop into a fine, sharp powder that can get everywhere.
- Use a container. If you’re snapping the tail, do it inside a heavy-duty plastic bag or a thick glass jar to contain the explosion.
- Check for "Self-Destruction." Sometimes, internal flaws cause these drops to explode hours or even days after they’re made. Never leave one sitting on a glass table or near your face without a cover.
The Prince Rupert Drop reminds us that strength is often just a balance of internal pressures. It’s a piece of 17th-century tech that still teaches us how to build better, stronger materials in 2026.
To see this in action, I'd recommend looking up the high-speed footage from the 2017 Purdue study or the various ballistic tests online. Seeing the "shockwave" of the fracture move through the glass at Mach 5 is something you won't forget.