Wiktionary
ninja rocks
n. (ninja rock English)
Wikipedia
Ninja rocks
Ninja rocks are broken shards of spark plugs formed by smashing the ceramic portion of the spark plug with a hammer or other large object. Since they can quickly and quietly fracture the glass side windows on most cars, ninja rocks have been used in " smash-and-grab" auto burglaries since at least 1995 (at least by Slovenian car burglers). They have no traditional association with the ninja or ninjutsu, only being named such due to their "silent but deadly" function in burglaries.