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. They have no traditional association with the ninja or ninjutsu, only being named such due to their "silent but deadly" function in burglaries and a superficial resemblance to the shuriken stereotypically used as a throwing weapon by ninjas.