My friends keep telling me that the porcelain part of a spark plug when hit againt a windshield will shatter it. Supposedly some chemical reaction with the window and something to do with the sweet spot. I personally think they are full of it, but let me ask the dope peoples here…
This is a post from just a little while back about the subject, which in turn has a link in it that references an older post about the subject.
Your friends are screwed. Tell them I said so.
And see this thead. In it, Cheesesteak has the likely answer. Note that this is more likely to work with side windows, rather than the windscreen (which is not normally tempered glass).
http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=120160
Baseball bats, tire irons and MagLite[sup]TM[/sup] flashlights are much more effective for shattering windows, and way more fun. Try one of those next time you really need to shatter a windshield.
Disclaimer: Don’t try this at home. Go down the street a few houses. 
A spring-loaded center punch will break tempered safety glass. In fact, anything that ruptures the surface of tempered glass will cause it to break up into little shards.
This type of safety glass is made by rapidly cooling the outside of a pane of hot glass, usually by air jets. This shrinks the outside faster than it does the inside and compresses the whole sheet which greatly strengthens it. But it also leaves the glass with a lot of internal stress so that if the outer “skin” is broken it disintegrates into those typical small bits.
biatchslap: In regards to your friend that told you about that chemical reaction and sweet spot crap, never ever beleive anything that they tell you again. Ever.
The spark plug ceramic method is used by thieves because of the fact that it makes relatively little noise. It does this because there’s a lot less force needed with the ceramic compared to a brick for example. Put simply, a brick breaks glass by forcing it to bend until it breaks. The ceramic is harder and stronger than the glass, so it will cause a fracture on impact that will continue to be transmited through the glass due to it’s amorphous structure.
Nice quick explanation about glass, glass-ceramics, and ceramics:
http://chemed.chem.purdue.edu/genchem/topicreview/bp/materials/ceramic4.html
Another, though less pertinent:
http://matse1.mse.uiuc.edu/~tw/ceramics/prin.html