Transferring money by text message is far safer and more convenient than cash. M-Pesa, as it is known, first took off in Kenya. The idea was to make it easier for small businesses to repay micro-finance loans. But, almost immediately, M-Pesa exploded into something far bigger - there are now 100 times more M-Pesa kiosks than ATMs in Kenya – and with far-reaching consequences, in many developing economies. Tim Harford describes how money transferred this way is easy to trace, which is bad news for the corrupt. And good news for tax authorities.