Just as remarkable as the power of mobility, over everything from love to learning to global development, is how fast it all happened. It is hard to think of any tool, any instrument, any object in history with which so many developed so close a relationship so quickly as we have with our phones. Not the knife or match, the pen or page. Only money comes close—always at hand, don’t leave home without it. But most of us don’t take a wallet to bed with us, don’t reach for it and check it every few minutes, and however useful money is in pursuit of fame, romance, revolution, it is inert compared with a smart phone—which can replace your wallet now anyway.