▁Category : 1 4 5 3 ▁death s ▁ ▁Category : 1 5 th - century ▁Mong ol ▁r ul ers <0x0A> </s> ▁Nin ja ▁rocks ▁are ▁broken ▁sh ards ▁of ▁spark ▁pl ugs ▁ ▁that ▁are ▁formed ▁by ▁sm ash ing ▁the ▁cer am ic ▁portion ▁of ▁the ▁spark ▁plug ▁with ▁a ▁ham mer ▁or ▁other ▁large ▁object . ▁Since ▁they ▁can ▁quickly ▁and ▁quietly ▁f ract ure ▁the ▁glass ▁ ▁side ▁windows ▁on ▁most ▁cars , ▁nin ja ▁rocks ▁have ▁been ▁used ▁in ▁" sm ash - and - gra b " ▁auto ▁burg l aries ▁since ▁at ▁least ▁ 1 9 9 5 . ▁They ▁have ▁no ▁traditional ▁association ▁with ▁the ▁nin ja ▁or ▁nin j ut su , ▁only ▁being ▁named ▁such ▁due ▁to ▁their ▁" sil ent ▁but ▁dead ly " ▁function ▁in ▁burg l aries . ▁ ▁How ▁they ▁work ▁Tem per ed ▁glass , ▁which ▁is ▁used ▁for ▁the ▁side ▁windows ▁of ▁most ▁vehicles , ▁is ▁manufact ured ▁with ▁an ▁extremely ▁high ▁surface ▁compress ive ▁stress ▁and ▁high ▁internal ▁tens ile ▁stress , ▁giving ▁it ▁strength ▁and ▁dur ability , ▁but ▁also ▁leading ▁it ▁to ▁ab rupt ly ▁sh atter ▁into ▁thousands ▁of ▁tiny ▁pieces ▁when ▁it ▁breaks . ▁ ▁When ▁thrown ▁with ▁moder ate ▁speed ▁at ▁a ▁side - window , ▁a ▁sharp ▁sh ard ▁of ▁the ▁exception ally ▁hard ▁al umin ium ▁ox ide ▁cer am ic ▁used ▁in ▁spark ▁pl ugs ▁focus es ▁the ▁impact ▁energy ▁into ▁a ▁small ▁enough ▁area ▁without ▁bl