Even the tiniest nick in the surface of a piece of tempered glass causes it to explode into small gravel-like chunks that don't have the sharp jagged edges of broken plate glass.
Back when I was a kid, a buddy and I were throwing pebbles at a transformer. His mom's 58 Chevy was parked in the line of fire, and one of the pebbles hit the back window. It immediately cracked into a million bits, but stayed in place. Then it slowly started falling apart, starting at the curved parts on each side. Pretty awesome. I think it was tempered. We told his mom that we had no idea what happened.
