Originally Posted by wiki.answers

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.