Developers who train machine-learning algorithms have found that it often makes sense to build toasters rather than wonder-boxes. That might seem counterintuitive, because the AIs of Western science fiction tend to resemble C-3PO in Star Wars or WALL-E in the eponymous film – examples of artificial general intelligence (AGI), automata that can interact with the world like a human, and handle many different tasks. But many companies are invisibly – and successfully – using machine learning to achieve much more limited goals.