A/N: This is a companion piece to Chemistry. Enjoy!

He did not like being confused.

Even so, like it or not, he was confused. Very confused.

There was a difference, as he knew very well, between reason and cause. He could understand the cause now, but the reason was beyond him. He knew that cause led to effect, he understood that. What he didn't understand was why this particular cause led to this particular effect.

That was the only thing he didn't know, and it drove him mad. He knew that she was the cause. He knew that whenever he looked at her, his hearts raced and he felt like he couldn't breathe. He knew that when she smiled at him, that cheeky smile with the tip of her tongue between her teeth, he felt like he was falling, or floating, or something. He knew that, but he couldn't figure out why!

Sometimes, he could feel her watching him. If he was close enough, he could taste the change in her, the adrenaline that pumped through her blood. When he looked at her, though, she looked away, a knowing smile lifting the corner of her lips. It was as if she knew how confused he was, and she somehow knew what he was trying so desperately to figure out. If anything, that made him even more crazy. How could it be that she, an ordinary human girl of nineteen years, could understand something that he, nine-hundred-year-old Time Lord and protector of the universe, couldn't make head nor tail of?

And yet, here they were: the ancient, clever, knowledgeable Time Lord, completely bewildered; and the young, mortal, human girl of average intelligence, who made sense of what had him at a complete loss. How did that happen? He tried to work out how it was that she had a complete understanding of the one thing that confounded him. It made even less sense than how somehow, impossibly, the Daleks always managed to survive. Compared to this, that was perfectly reasonable.

He tried to figure it out. In his mind, it all simmered down, all the parts of the equation merging together, the terms combining, until just one word was left. The only reason he could think of. A silly reason, but a reason, which was what he'd been looking for. Could that be it, then? Was that the answer? Could he really, actually be...?

Impossible.

But then, he loved impossible.

A/N: Here it is, Chemistry from the other side. Like it? Dislike it? Tell me! Review!