Camouflaged personnel with no insignia, protesters bundled into unmarked vans: the President Donald Trump's plan to put federal officers into American cities is a worrying political ploy. Our annual Big Mac index examines which currencies are over- and undervalued; we take a meaty look at what burgernomics reveals. And Indian scientists simultaneously solve a water problem while taming a fire problem. For full access to print, digital and audio editions of The Economist, subscribe here www.economist.com/intelligenceoffer