"It's a screwdriver," clarifies Andrew, stopping to grab a bag of crisps off the shelf this time. "It's sonic. Use the right frequencies in the right concentrations and you can do anything with sound. Well, almost anything. It doesn't do julienne fries or anything like that."

He examines the bag, then puts it back upside-down. "D'you know, I can't remember how I came to be in an underwater city. Can you?"