Prince Rupert's Drops
Drops of molten glass, consolidated by falling into water. Their form is that of a tadpole. The thick end may be hammered pretty smartly without its breaking, but if the smallest portion of the thin end is nipped off, the whole flies into fine dust with explosive violence. These toys, if not invented by Prince Rupert, were introduced by him into England.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894