It opens by asking you to drag a finger across a window, then across your screen, and explains the difference: a non-stick coating one molecule thick that wears away a little every time you touch it — which is also why a face-down phone slides off the faintest slope. Fourteen layers follow, down through the potassium-bathed glass Jobs demanded after his keys scratched the prototype. Read it on a phone; that’s the joke and the point.