Title: Subtle Chemistry

Rating: K

Summary: The Doctor (10) tries to explain to Rose just why he likes Chemistry so much.

Disclaimer: Not mine, as you've probably guessed.

A/N: Thank you, thank you, thank you to saganamidreams for being my beta, not just on this but also on the stupid epic which must be killing her. So thanks to her. The help is fantastic. And you can also probably blame her for this fic because it stemmed from one of our online conversations.

Anyway, I apologize if you're not as into Ten as I am. This is a Ten/Rose fic, but not overly so. Just a bit of fun. Please tell me what you think. On with the fic…

"Since when are you such a science nut?" Rose asked from her position at the TARDIS's console. Ever since he'd regenerated, he'd become fascinated with tinkering with chemicals.

The Doctor looked up, slightly annoyed at her having caused his distraction. He was having major issues with fixing his beloved ship and, while not quite ready to start hitting various parts of the console with a mallet (that would have been pointless and cruel), he was pretty close to just giving up and finding an adventure in whatever space and time the TARDIS decided to take them; steering was for the boring anyway. He climbed out from the crawl space, ignoring Rose's question for the time being, and looked at the monitor to work out just where in space and time they were.

Ah, the end of the Precambrian era on earth. A time when rocks and single celled bacteria ruled the planet. How fun. He pulled a face at no one in particular, turned to Rose and decided that, with nothing to save Earth from and nothing on it really worth saving, talking to Rose was his best option.

"I've lived nine hundred years," the corner of her mouth twitched up at the Scottish accent she was still getting used to, "I've picked up quite a bit of science along the way."

"The old you never used to be this obsessed with science. I didn't even know the TARDIS had a lab, now it's the first place I go to find you." Watching him stare at the monitor in front of him, brow creased low and his hair falling shaggily over his forehead she couldn't help but quip, "And you used to be a lot better at fixing the TARDIS…A lot better. Where's that ability gone?"

His neck cracked as he whipped his head around to look at her, defensively explaining, "He used to hit her, with a mallet. It wouldn't have even done anything. That's probably the reason she's broken now." Looking back to the monitor, he tried to concentrate harder on working out what was wrong.

She had to keep bringing up his old self. Was it possible for him to be jealous of himself? Because that's certainly what it felt like. The old him had liked Rose. And, even completely ignoring all the past feelings, he liked her, too. Ignoring all feelings whatsoever, just based on what he knew she'd done for him, he couldn't help but like her. People who sacrifice themselves for you, whether they remember it or not, are instantly likable, he reasoned. And besides that, getting to know he, from a different perspective, he found that he enjoyed her personality, her presence. Glancing up he realized he'd gotten lost inside his own head again and she was standing, arms crossed in front of her, looking unimpressed.

"Nine hundred years of chemistry I've got. Always had it, just because the old me didn't think it was useful doesn't mean that I don't. How can you not love chemistry?"

She didn't look convinced but she liked him like this, reminded her of the old Doctor but in a different way, he was just as animated but with his hands more than his face; just as excited, but about different things. "Chemistry is boring. You spend two hours getting out beakers and vinegar just so you can make a few bubbles. Don't you think knowing how to fix the TARDIS is a slightly more important thing to be good at?"

He glared at her but the upwards curve of his mouth made it clear he wasn't serious. "Chemistry is what makes everything work. Everything. You, me, the entire universe is based on how tiny invisible particles react, chemically. How can you not love it?" Turning around, he leant back on the console, deciding he'd simply find the manual later and fix it while she was sleeping.

"It's boring," she said resolutely. Was it a good thing or a bad thing that not four months after the regeneration she already felt completely comfortable with him and when he talked about the things he cared about, as stupid as they might be, she felt familiar tingles running up and down her spine?

"It's not boring…It's subtle."

She grinned, moving to lean on the console beside him and leaning forward so that she could reach out and fiddle with the left pocket of the Doctor's long jacket as she'd recently taken to doing. He continued to lean with his back against the console; smiling and angling his head ever further to look down at her and await her response.

"I don't like subtle. People who like things to be subtle read books and listen to symphonies, they don't run around through time and space saving the universe."

When he'd regenerated, she'd missed the banter, but she had noticed it creeping back into their conversation, bit by bit, a month or so ago. Now it really was back: completely different and yet so similar. Outside of her, the tangible part of it was different, but how she reacted, how she felt, was exactly the same. "In fact, subtle is a nice way of saying boring," she finished, her tongue slipping between her teeth as she tried, but failed, to keep the huge grin from breaking loose.

He paused for several seconds before saying anything, the whole time his eyes locked with hers. "Chemistry is a million little subtleties which eventually just have to ignite and explode."

Mentally that simple sentence made her stagger and she faltered for a minute. Why it affected her she wasn't sure, so she pushed for more, "What's that s'posed to mean?"

"Exactly what I said. No matter where chemistry goes, or who controls it, it begins with one subtle little thing, an atom of oxygen. You add another subtle thing, as boring as just another atom of oxygen. Do it a billion times, grouping them in twos. But do it perfectly because if you do it wrong just once, make one with three atoms or just one, it will all go wrong. Pack them tightly until all these tiny little atoms, pairs of oxygen atoms, subtle, you might say boring, are bouncing off each other madly, throwing energy everywhere but to you or me, they are still completely invisible. Then…" he paused for effect, astounded she hadn't wandered off or fallen asleep and silently chuffed that he just might have converted someone, and not just any someone, but Rose, to chemistry. "Light a match…Boom."

He still took her breath away, and just talking about chemistry. Better to have him think it was the chemistry effecting her rather than give him an even bigger ego. A minute after he'd finished his little speech, the small portion of her brain which wasn't concerned with the grin the Doctor was sending her but had been working vigorously at formulating a comeback, finally reconnected with her mouth. "Some things really will never change."

The grin fell away, replaced by a look of confusion. Before he could question her, she said, "All you're really interested in is blowing things up." And with that she walked off, hoping she'd be able to catch her breath and rewire the intelligent part of her brain back into the rest of her body before her next run in with the Doctor.

He just stayed where he was, watching her as she strutted away, obviously pleased with herself. Perhaps he'd have to make this an ongoing thing, a new game to play. He tried to think of a chemistry experiment that was interesting but didn't blow up. He came up with several, though quite a few less than the ones that flittered across his thoughts that were sure to impress but, as a requirement of their impressiveness, had to make a crater in something. Perhaps some sort of a fluorescence thing…glow in the dark paint…that could be interesting.

The TARDIS jolted beneath him and he started, once again shaken from his silent interlude. With a hopeful expression he looked at the monitor again. How exciting, they'd jumped into the early Cambrian era. That meant worms and snails. Fantastic. Turning and shoving his hands deep in his pockets, he walked off in search of the library and in it, he hoped, the 'Idiot's Guide to Driving a TARDIS'.