Why can one small piece of porcelain shatter auto glass?

Factual Questions
1

This post about car windows breaking with no apparent cause, reminded me of something else.

You take an old car spark plug, and smash it with a hammer until the porcelain insulator breaks up.

If you toss one of these porcelain fragments at a car window, the window instanly cracks up over its entire surface, and then, any slight disturbance causes the glass to fall out.

I remember in high school kids would mention this, but I didn’t believe it. At the local junkyard, I tried this on a car that was about to go into the crusher anyway, and it does indeed work as described. It seems to take as many as two or three porcelain-tossing attempts, but you don’t even have to throw it very hard.

Is spark plug insulator material really porcelain?

Why/how does this work?

2

It sort of surprises me that this would actually work, I would think that the piece of porcelain (I think the insulator is porcelain, which has been used as an electrical insulator for a long time) would be too small to actually do anything. Here’s my theory anyway:

Porcelain is a harder substance than auto glass (7 vs 6 on the Moh’s scale) and can easily scratch the surface, similar to how a glass cutter scratches a glass surface. Because auto glass is often under high stress, a significant scratch or chip can cause the entire piece to shatter.