Which are the world’s biggest cities, and what are their populations? Two simple questions that we discover are surprisingly difficult to answer. Plus, has the world got heavier or lighter since the industrial revolution? It’s a question posed by a More or Less listener that got us wondering, too. Dr Chris Smith, part of a group of Cambridge University researchers, known as the Naked Scientists, reckons he’s worked out the answer. This programme was originally broadcast on the BBC World Service.