A weak currency, despite its appeal to exporters and politicians, is no free lunch. But it can provide a cheap one. In China a McDonald’s Big Mac costs just 14.5 yuan on average in Beijing and Shenzhen, the equivalent of $2.18 at market exchange rates. In America the same burger averages $3.71. That makes China’s yuan one of the most undervalued currencies in our Big Mac index, which is based on the idea of purchasing-power parity. This says that a currency’s price should reflect the amount of goods and services it can buy. Since 14.5 yuan can buy as much burger as $3.71, a yuan should be worth $0.26 on the foreign-exchange market. At just $0.15, it is undervalued by about 40%. The tensions caused by currency misalignments prompted Brazil’s finance minister to complain last month that his country was a potential casualty of a “currency war”. The Swiss, who avoid most wars, are in the thick of this one. Their franc is the most expensive currency on our list.