A nine-year-old child announcer has been recruited on the London Underground. The idea is that her voice will surprise passengers, so they listen to her safety message. It’s an example of nudge theory in action, the art of subtly persuading large numbers of people to change their behaviour, by adjusting their environment. People Fixing the World also visits a university campus, which is nudging its students with a subtle price change, encouraging them to use fewer disposable coffee cups. Presenter: Harriet Noble Reporter: Dougal Shaw Photo Caption: Nine-year-old announcer Photo Credit: BBC