"Sir? Sir?" The Doctor glanced around, as if he was trying to work out who Billy was actually talking to, then pointed a finger at his own chest. "Me? I'm not a sir. Well, not really. There was the time I met Queen Victoria ..."

If he'd had a companion with him, they would have chosen that moment to hit him on the arm and bring him back to the problem at hand. He didn't, though, so the Doctor had to manage it himself.

"Just call me the Doctor," he finished, before starting to jump from square to square to answer the second part of the question. He moved with such enthusiasm that it seemed almost random, but he knew exactly what he was doing and why.