"I've never even heard of that," the Doctor said, incredulous. "And I was taught recreational mathematics since I was young!"

He was also a little ego-bruised that Jack had figured out the puzzle before he had. All the same, he hopped four squares and landed one foot on the dark red square, his other foot hovering over the electrified squares around him.

"Make sure you start following me," he called back again. "Don't want them switching this up once I've got across."