Your phone is in fact covered with some very hard, very strong glass that's only 0.8 mm thick. Yet it takes 30 times more force to scratch the iPhone screen than it does a piece of plastic, according to Apple. And under perfect conditions, the latest chemically tempered Corning Gorilla Glass — which is more likely than not covering the front of the new phone in your pocket — can withstand around 100,000 pounds of pressure per square inch. Or in English, a 1.18-pound iron ball dropped from six feet.

Building glass that tough is an arduous process. A 400-degree Celsius bath of molten salt replaces the small sodium ions — the tiny particles that make up aluminosilicate sheet glass — with larger potassium ones that “stuff” themselves onto the surface, for increased compression. This makes the glass harder and more resistant to deeper scratches. Jets of cold air are then blown across the top of the glass, which allows the outside to cool and harden faster than the inside, giving it lots of compression on the surface, but tension in the middle. The push and pull between the two is what makes tempered glass so strong.