The tough-yet-fragile physical properties of the tadpole-shaped pieces of glass known as Prince Rupert’s drops have puzzled physicists for as long as, well, there have been physicists. Bash the head with a hammer, and a drop gets barely a scratch. But break off its thin tail, and it shatters into fine powder. Researchers long ago realized that the strength of the drops—named for Prince Rupert of Bavaria, who presented five of them to Britain’s King Charles II in 1660—has something to do with stresses in the glass created when a drop is made by letting a blob of molten glass fall into water, so that it cools rapidly. Twenty years ago, a pair of researchers took high-speed video of a drop disintegrating showing that, when the tail is broken, cracks propagate along the stressed glass at more than 1700 meters per second. Those researchers have now teamed up with others to study the stresses in the head of a drop. Using a technique called integrated photoelasticity, they immerse the drop in a liquid and shine polarized light through it. Stressed areas propagate the polarized light differently, and by processing the light with techniques similar to those used for medical computerized tomography scans, the researchers were able to map out the different layers of stresses inside the drop. The extraordinary strength of the head, they reported in Applied Physics Letters, comes not from tensile, or pulling, stress—as researchers have long believed—but from compressive stress. The team measured compressive stress in the drop’s head equivalent to more than 4000 times atmospheric pressure—making it as strong as some grades of steel. Tensile stresses, which exist in the tail and interior of the drop, tend to propagate cracks, but the overlying compressive stress in the head suppresses them. The compressive outer layer shields the head from hammer blows. But a snapped tail lets the cracks race through the drop, and although the compressive outer coat slows the cracks down, by that stage it can’t stop them.