"This could not be a layer above or below," says Kell with a thoughtful frown creasing his brow. "One may not go from White to Grey, for example, never around, only through. If I moved from Red to another, it could only be White or Grey."

It's beginning to hurt his head, in truth. "Another napkin perhaps, as you say," he concedes. "But if there's another world beneath us, I can't reach it without a token."