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From The Collaborative International Dictionary of English v.0.48:
Rupert's drop \Ru"pert's drop`\ A kind of glass drop with a long tail, made by dropping melted glass into water. It is remarkable for bursting into fragments when the surface is scratched or the tail broken; -- so called from Prince Rupert, nephew of Charles I., by whom they were first brought to England. Called also Rupert's ball, and glass tear. [1913 Webster]