Car thieves have come up with an ingenious way to enter cars by breaking the side windows with something you would never imagine could be used as a tool: spark plugs. By smashing or crushing the alumina insulator most commercial spark plugs have, the thieves have an instant tool that will exploit the stress present in the side windows of most cars, instantly shattering them. “Ninja rocks” — as they’re called — have no association with the ninja other than being named such due to their “silent but deadly” function in burglaries. Tempered glass, which is used for the side windows of most vehicles, is manufactured with an extremely high surface compressive stress that gives it strength and durability against shocks and blunt impact. These physical stresses in tempered glass makes it vulnerable to ninja rocks. The glass shatters into thousands of tiny pieces, reducing the risk of laceration. In California and the state of Washington, ninja rocks are explicitly listed as burglary tools.
