<P> Ninja rocks are broken shards of spark plugs that are 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 . They have no traditional association with the ninja or ninjutsu , only being named such due to their `` silent but deadly '' function in burglaries . </P>
<P> Ninja rocks are broken shards of spark plugs that are 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 . They have no traditional association with the ninja or ninjutsu , only being named such due to their `` silent but deadly '' function in burglaries . </P>
what part of a spark plug breaks glass
what part of a spark plug breaks glass
the ceramic portion
the ceramic portion