Prince Rupert drop is a glass structure of which the head is unbreakable even when hit with a bullet, while the tail is such that merely tapping it shatters the entire structure.
In 1994 materials scientists Srinivasan Chandrasekar from Purdue University and M. Munawar Chaudhri from the University of Cambridge watched it shatter with a high speed cam. What they found was that breaking the tail appears to send a line of cracks running down the length of the drop at speeds of up to 1,900 m/s (6,200ft/s).
A couple of months ago some scientists lowered a drop into a liquid with the same refractive index as the glass. What they found was that the outer layer of the glass had astonishingly high compressive stress.
In their words , the secret behind PRD's functioning is that "In order for a PRD [Prince Rupert's drop] to disintegrate catastrophically, it is necessary for any cracks, induced by the compression process, to enter the tension zone in the head of the PRD".
That's why they're so easy to break at the tail end. Once a crack gets through the compressed skin into the zone that's held under tension, it's game over.