Ninja rocks
Ninja rocks is a colloquial term for an improvised weapon or tool consisting of the extremely sharp porcelain or ceramic shards recovered from smashing or crushing the alumina insulator of a commercial spark plug. When thrown, ninja rocks are known to exploit the tensile stress present in the side windows on most cars in order to instantly shatter them, providing a quick and quiet alternative to other window-smashing methods and making ninja rocks ideal for emergencies or "smash-and-grab" auto burglaries, having been used in the latter function since at least 1995.