Prince Rupert’s Drops

Drop a bit of molten glass into a bucket of cold water and you’ll produce a teardrop-shaped bauble with a long tail. Surprisingly, you can pound on the bulbous end with a hammer without breaking it, but snipping the delicate tail causes the whole drop to explode. The water hardens the outer shell before the interior has cooled and contracted, so the finished drop carries high compressive stresses on the surface and tensile stress at the core.

The drops were known in northern Germany as early as 1625 and distributed through Europe as toys, though the underlying principles were not well understood until the 20th century. Prince Rupert of the Rhine (1619-1682) did not discover the drops but was the first to bring them to England, where Charles II delivered them to the Royal Society. The anonymous Ballad of Gresham College (1663) immortalizes the experiments that followed:

And that which makes their Fame ring louder,
With much adoe they shew’d the King
To make glasse Buttons turn to powder,
If off the[m] their tayles you doe but wring.
How this was donne by soe small Force
Did cost the Colledg a Month’s discourse.