Tiny shards of spark plug porcelain have small hard points which allow them to easily find a breaking point in glass. They can also be used to break the side window of vehicles. Spark plug porcelain fragments are sometimes referred to as ninja rocks because of their ability to break glass with ease.
Spark plugs are made of aluminum oxide ceramic, which is exceptionally hard. The impact of the shard against the glass creates a tiny structural failing, which rapidly spreads the tension through the rest of the screen, causing it to break. Although effective, spark plug fragments will not work all the time. Spark plug fragments may not break glass if they are not sharp enough or thrown with too little force. Spark plug porcelain shards are also less effective on windshields because these come with a special laminate feature that reinforces their resistance to impact.
In some states, broken shards of spark plugs are considered a weapon. This is because they are sometimes used by burglars to break into vehicles. In the state of California, carrying spark plug chips or pieces is considered a misdemeanor. Individuals caught carrying spark plug chips run the risk of paying a potential $1,000 fine or receiving a 6 month jail term.