The reason spark plugs are so effective at breaking the glass is because a ceramic spark plug is considerably harder than glass, glass only hits around 5.5 on the mohs hardness scale, whereas ceramic is around 9 (lead is around 1.5 so ceramic is MUCH harder than glass even though glass seems very hard) Because of this ...

Can a spark plug break a window?

A: Yes, a spark plug thrown at a car window will break it. So will a rock, a good-size ball bearing or a ball-peen hammer. The side widows are made of tempered glass that, while strong, can be broken with a strong blow. The glass will then shatter into cubes instead of shards so you are less likely to be seriously cut.

What happens if a spark plug breaks while driving?

Do NOT drive a car with a piece of sparkplug inside the engine. It can (and likely will) lead to serious damage (up to a catastrophic engine failure). Get your car towed to a service and have the pieces of the broken plug removed. Additionally, get the oil changed (probably with a flush too, to remove all the debris).