Jack was almost confused at first; his mind not making the right connections. But then he understood and he nodded just a little.

"Small enough," he said quietly. Funny, he'd spent so long in some of the tightest spaces within the TARDIS while fixing her up and that had been fine, but a dark little room like this and he was suddenly underground again being covered with dirt.

He turned his head slightly towards him, looking at him in profile.

"I don't think anyone does know," he said. "Everyone just makes it up as they go along. And if anyone can do that it's you." He took a long breath. "If we all dwelled on our mistakes we'd--" he stopped and laughed slightly, ironic. "Well, we'd be me."