There is an audacious economic phenomenon happening in China. It has nothing to do with debt, infrastructure spending or the other major economic topics du jour. It has to do with cash — specifically, how China is systematically and rapidly doing away with paper money and coins. Almost everyone in major Chinese cities is using a smartphone to pay for just about everything. At restaurants, a waiter will ask if you want to use WeChat or Alipay — the two smartphone payment options — before bringing up cash as a third, remote possibility. Just as startling is how quickly the transition has happened. Only three years ago there would be no question at all, because everyone was still using cash.