"I'll figure it out," the Doctor said, seriously. "It's like a task."

He smacked his lips and considered. "Right, not alien, so that's right. Not Winchester that's right. Part of it. Genetic predisposition to alterations? No, no, that's not close enough. Clone, maybe? Taken from Dean's genes? No, no, his sense of smell isn't nearly strong enough. Maybe...time traveler? Time Agent?"