They do not break(shatter) laminated saftley glass like windshields, I think you mean tempered safety glass as used in doors. It has to do with hardness. Glass is actually "harder" than iron on the Mohs scale, and spark plug ceramic (technically called "aluminum oxide ceramic") is much harder than glass. Aluminium oxide ceramic actually rates a 9 on the Mohs scale; diamonds are 10, glass is 6.5, and iron is 4.5. That's the key to the whole thing, and why it's surprisingly hard to break a window with a hammer and surprisingly easy to break it with a small, light little shard of innocent white spark plug ceramic. (via http://www.ridelust.com/obscure-burglary-tools-of-the-day-ninja-rocks/)

Other answers (not accurate, read comments below) include:

- A spark plug is heavy this would be no mystery but if you mean how does the ceramic from a sparkplug break safety glass than your question will be answered.

- The ceramic from a sparkplug attracts negative electrons (Negative Static) A cars side or rear window (Tempered Glass ie safety glass) has positive electrons (Positive Static) If one were to throw a peice of sparkplug ceramic at a car window the first part of the ceramic that touches the window creates a miner spark explosion which temporarly weakens the glass to an extreme point than the rest of the weight of the ceramic breaks the glass.

- You can shatter a car window with a piece of ceramic from a sparkplug about the size of your pinky finger nail.

- Last I checked, POSITIVE electrons are positrons, a component of antimatter...rather overkill to cause a nuclear event just to break a window. Rethink your physics.---Although the answer is not correct... a positive electron can also be considered a hole, or rather a lack of an electron... not necessarily your exotic anitimatter.