The entire Tube network is down, they're told. And they're the only people with any hope of getting it back into operation. If the performance of the schoolchildren involved is anything to go by, one day they might really be relied on to maintain the transport systems of the future. But for now this is just an exercise, albeit one they take very seriously. It is just one of a a huge number of "Field Trips" happening at Apple Stores across the country as part of the iPhone maker's support of the government's Year of Engineering scheme.