Prince Rupert's drops are toughened glass beads created by dripping molten glass into cold water, which causes it to solidify into a tadpole-shaped droplet with a long, thin tail.
It's head is extremely strong but it's tail is extremely fragile. The head can withstand a bullet, but even a slight hit on the tail can break the whole structure.
After the glass is dropped into the water, the outside layer of glass cools while the inside stays relatively hot. The outer layer of glass shrinks as it cools and forms a solid shape.
When the glass core of the drop eventually cools, the molecules inside have no where to shrink to because the outer layer is already set, so they pull toward each other, creating a super high tension inside the bulb, which eventually hardens. This tension is released when the tail is even slightly tweaked, it releases a cascade of energy that propagates the entirety of the tail and bulb, exploding it outward.
@Sci_Matrix