Bounce, then break: Why it sometimes takes two drops to break an iPhone screen
Bryan Gardiner, writing for WIRED Science about Corning's Gorilla Glass, the tempered material–first invented in the '50s!–that covers iPhones and other devices (emphasis mine):
All this destruction and controlled mayhem has paid off. Compared with the first version of the glass, Gorilla Glass 2 is 20 percent stronger (a third version is due out early next year). The Corning composition scientists have accomplished this by pushing the compressive stress to its limit—they were being conservative with the first version of Gorilla—while managing to avoid the explosive breakage that can come with that increase. Still, glass is a brittle material. And while brittle materials tend to be extremely strong under compression, they’re also extremely weak under tension: If you bend them, they can break. The key to Gorilla Glass is that the compression layer keeps cracks from propagating through the material and catastrophically letting tension take over. Drop a phone once and the screen may not fracture, but you may cause enough damage (even a microscopic nick) to critically sap its subsequent strength. The next drop, even if it isn’t as severe, may be fatal. It’s one of the inevitable consequences of working with a material that is all about trade-offs, all about trying to create a perfectly imperceptible material.
That explains so much.