system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: Why will a broken piece of ceramic from a spark plug, break a car window, easier and far more efficiently than a hammer?
gpt
Car windows are made of tempered glass. Tempering is a process during which glass is cooled very rapidly by quenching in water. This causes extremely high internal stresses in the glass, as the outside shrinks and solidifies first. Basically, the glass is pulling on itself very tightly all the time. This gives it the strength to resist hammer strikes and the hardness to resist scratching by most materials, but also means that if a flaw is introduced, such as a scratch, the internal stresses will release around it and cause it to grow rapidly, releasing even more stress, until the entire pane shatters. 

The piece of ceramic has higher hardness than even tempered glass and can easily scratch it, starting the chain reaction.