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. Functionality Ninja rocks take advantage of the physical properties of tempered glass, disrupting surface compressive stress and causing the glass to shatter. Tempered glass, which is used for the side windows of most vehicles, is manufactured with an extremely high surface compressive stress and high internal tensile stress. This gives it strength and durability against shocks and blunt impact. When the glass breaks (such as in a serious vehicular collision) the internal stresses present in the pane cause the entire pane to shatter into thousands of tiny pieces. This reduces the risk of laceration one might otherwise face when using 'normal' glass, and is an essential safety feature in vehicular design. It is these physical stresses designed into tempered glass which make it vulnerable to ninja rocks. Made of shards of aluminium oxide ceramic, ninja rocks are very hard, and very sharp. When thrown at tempered glass, the ninja rocks' sharp, hard point focuses impact energy into an incredibly small area without blunting. This disrupts the glass surface compressive stress at the point of impact, subsequently releasing the internal potential energy within the stressed pane, shattering the glass. To be effective, a ninja rock needs to be sufficiently sharp, impact the glass on that sharp point, and impact it with sufficient force. Thrown ninja rocks may often fail to shatter tempered glass if one of these conditions is not met. Ninja rocks are ineffective against windshields, as these are made of a laminated type of safety glass, and designed not to shatter. Legal status United States of America California In California, since 2003, ninja rocks have been explicitly listed as burglary