Mike shows how porcelain from a spark plug can break glass with ease. Then tries to do the same with a rock. Why doesn't he just use the hammer? Because this is not a video on how to break glass ...
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.
As the article you linked to mentions, spark plugs are made out of fairly hard material, and if broken spark plug insulators (the ceramic part) are used to break into a car, their jagged, sharp edges may be a factor as well. posted by Vidiot at 7:03 AM on November 30, 2004
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.
In order to break a car window's tempered glass, the porcelain insulator inside of a spark plug can be used. Unglazed porcelain is rated 7 on the Mohs scale of mineral harness, while glass is rated 5.5. To shatter a car window with a spark plug: Obtain the spark plug's insulator; Spark plugs generally use a porcelain insulator.
Edit: Several people have suggested that the charge of the glass and the ceramic from the spark plug create a spark that breaks the glass in an explosive way. This is a myth. The only charge of any significance here is the one you'll pay to replace your window.
That’s where this life hack for how to break a car’s window is not only useful but it could save a life. National Geographic host Brian Brushwood explains in the video below how a simple spark plug can shatter a car window more easily than a hammer. Car windows are made from tempered glass and are designed to resist blunt force.
The chances of that hitting and causing the break versus the actual metal nut or metal shell hitting first and ultimately causing the break is very slim. I have never been sure as to why people claim a spark plug does a better job at breaking a window versus just a small piece of metal. Like I mentioned above, a metal slug.
If you're stuck in a sinking car, or outside your car and desperately need to get inside, you have plenty of ways to break that window, but one of the best might be right there in your car anyway ...