Glass cup shattered on its own?

One man voletball 2.28.2012  6:15
Glass cup shattered on its own, anyone have an explanation?
A glass cup was sitting on the table. Not sure for how long but not too long. The Air Conditioner was on and I accidentally tapped it lightly and it shattered completely. This was totally freaky, and was only wondering if anyone had an explanation to this.
A few things could have happened depending on the type of glass. If the glass was cold and exploded into sharp slivers, it was probably Corelle or a similar glass. These are thin but strong. The strength comes from rapid chilling from the molten state. The surface hardens, so when the interior cools and contracts it draws the surface tightly inward. It's that tension that makes the strength, but there's a lot of residual stress--it's a tightly bound spring. If the surface is broken, that stress is released all at one and they can really explode.