It’s a question that has baffled scientists for more than 400 years: why are tadpole-shaped pieces of glass, called Prince Rupert’s drops, so strong?

The head of the drop can survive bullets, hammer blows and even hydraulic presses, but tweak the tail and the entire structure will instantly disintegrate.

Now scientists have discovered why.

Researchers used a microscope to map the stress distribution of the drops, which form when molten glass is dropped into water.

Their study revealed that the head of the drop has a super high surface compressive stress, withstanding 7,000 times atmospheric pressure.

Despite the surface compressive layer only making up a tenth of the diameter of the head of a drop, they have a very high fracture strength. 

To break the drop, it’s necessary to create a crack that enters the interior tension zone. Doing so is difficult as cracks tend to spread across the surface, rather than down into the vulnerable zone of the drop.

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However, disturbing the tail allows the crack to enter the tension zone much more easily. Dr M Munawar Chaudhri, a scientist at Cambridge University, told Phys.org: “The work has fully explained why the head of a drop is so strong. I believe we have now solved most of the main aspects of this area.”

If you want further proof of the drops’ strength, check out this YouTube video, which shows bullets shattering as they hit the head of the drop. 

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