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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).〔(Dark Side of Tinkering by Edward Tenner 2001 US News and World Report Dec 24 2001, reprinted in 2003 ''Knowledge, Technology, & Policy'' vol 16 (2) 42-44 )〕 They have no traditional association with the ''ninja'' or ''ninjutsu'', only being named such due to their "silent but deadly" function in burglaries.
==How they work==
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, giving it strength and durability, but also leading it to abruptly shatter into thousands of tiny pieces when it breaks. When thrown with moderate speed at a side-window, a sharp shard of the exceptionally hard aluminium oxide ceramic used in spark plugs focuses the impact energy into a small enough area without blunting to initiate cracking, releasing the internal energy and shattering the glass.〔(National Geographic demonstration video )〕 However, ninja rocks are ineffective if the shards are insufficiently sharp, thrown with too little energy, or thrown against windshields, as these are made of a laminated type of safety glass, and therefore do not shatter.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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