In 1519 Hernán Cortés and his conquistadors invaded Mexico, hitherto an isolated human world. The Aztecs, as the people who lived there are known to posterity, quickly noticed that the aliens showed an extraordinary interest in a certain yellow metal. In fact, the aliens seemed to never stop talking about it. The natives were not unfamiliar with gold – it was pretty and easy to work, so they used it to make jewellery and statues, and they occasionally used gold dust as a medium of exchange. But when an Aztec wanted to buy something, he generally paid in cocoa beans or bolts of cloth. The
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