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Cobalt, the metal in magnets and phone batteries, is synonymous with the colour blue. But what exactly are magnets, how do they work and where are they used? And is some of the cobalt being mined by children? Presenter Laurence Knight hears from chemistry professor Andrea Sella of University College London why a permanent magnet is like a flock of birds. He travels to Arnold Magnetics near Sheffield where manager Martin Satyr explains how magnets are used in everything from recovering the heat energy from sports car engines to recycling your trash.

Also, Mark Dummett of Amnesty International, back from Katanga in southern Congo - source of half the world's cobalt - tells of his concerns about the conditions in which artisanal miners work, including children. And, David Weight of the Cobalt Development Institute explains what the industry is doing to ensure it knows where its cobalt is coming from.

(Photo: Andrea Sella picking up lumps of cobalt with a magnet)