Title: The Sweetness Triangle
Part Three in the Chemistry series
Part One: Subtle Chemistry
Part Two: Chemical Symmetry
Rating: K+
Summary: The chemistry continues.
A/N: Subtle Chemistry was supposed to be a one off. Not the start of a series…but how can I resist, with so many wonderful, wonderful reviews. Not to mention sangamidreams, blame her. She's the beta who manages to beta things instantaneously. Anyway, here is the next installment, longer, with more advanced chemistry…that sentence works on so many levels.

Bah. Anyway, if I get the same level of feedback as what I got for the last two, I shall be incredibly pleased. It's just way to wonderful. But yes, if you still like it, I shall write another. I'm really enjoying it.

"Teach me something else about chemistry?" It was phrased like a hopeful question because that's what it was.

Ignoring her, he decided, was his best option. He did it a lot anyway, just pretended he hadn't heard until he'd worked out the perfect answer and could just slot it in ten minutes later. It made her angry; she'd pout and fiddle while he ignored her, just like she was doing now. But what was he supposed to do? They'd come to a silent mutual agreement to cool off on the chemistry ever since the phosphorescent gas caused a little too much friction. That had been four days ago.

And now, here Rose was bringing it back up with all the subtlety he should have expected. He stared harder at the TARDIS manual, trying to follow the troubleshooting instructions which appeared to have been written by an Ikea fanatic on crack. The original problem, when they'd been stuck in Precambrian Earth, had fixed itself and the ship had been right for a time. They'd even managed to get to Palladium Two Tetraaqua and save the Vusoo from certain annihilation. However, about to set course to the DiAntimony Shopping Mall (Rose thought this was for shopping; he knew better) the Doctor had noticed several of the levers were refusing to budge. Now they definitely needed the manual.

It had been he who found it, nestled in his library, inexplicably stuffed between his own original edition of 'The Dead Sea Scrolls' and the July 2092 edition of 'Empire' magazine. Dismayed, he'd found it covered with sparkles and glitter and with bold type stating for the entire world: "Idiot's Guide to the TARDIS." He remembered one of his earlier selves doing that; had thought it funny at the time. Not so much now with Rose just two bookshelves away. Luckily he'd managed to hide it long enough from Rose to rebind it in brown leather.

Behind him, Rose stomped her foot, much like a child and demanded, "Why do you always pretend you can't hear the question when you so obviously do?"

Another one to ignore…he realized that probably wouldn't work two seconds too late when she edged her way between him and the console, making it both impossible to continue work and also rather difficult not to look at her.

When he did, he saw the impatience and the pout and very quickly gave in, opting for the first question she'd asked because he didn't really have an answer for the second. "What sort of chemistry would you like to discover?" Perfect, asking the question also gave him time to step back, shove his hands and sonic screwdriver into his pocket, and make absolutely sure that he was no longer able to feel the heat of her body.

The look on her face told him that she thought he'd just conceded a point by stepping back. Ignoring his competitive streak, he tried to concentrate on her words. "I dunno, anything. You haven't been disappointing yet."

Didn't she know the chemistry was everything? It wasn't as though he could choose an area at random. She had to be specific, so he prompted, "Organic or Inorganic?"

"What's the difference?" she asked, leaning languidly back against the console.

Briefly he wondered just what level she was operating at. She wasn't stupid, but chemistry certainly wasn't one of those intuitive subjects. You either knew it or you didn't. Still, it could be taught. "Organic encompasses most of the naturally occurring chemistry, particularly living things. Base atoms are oxygen, carbon and hydrogen." He smiled at how interested she looked, her head tilted (this was fast becoming his way of judging her true interest) and her brow creased as she took the information in. "Inorganic is synthetics, metals and halide based molecules."

"Synthetics as in…" she just wanted more explanation. Though, hypothetically, would it be wrong if him talking about chemistry were just a tiny little bit of a turn on?

Hypothetically?

"Polyester, clothing dyes, plastic, that kind of stuff."

"Organic then."

Stifling the look of pride (he was an organic guy, too) the Doctor thought about it again; organic chemistry still encompassed a lot. "Bio-organic or Enviro-organic." The look she sent him, brow creasing a little more, tongue darting out as her logic did its best to dissect and understand his language, caused him to elaborate. "Bio is if you want animal or plant chemistry. Environmental is for dirt, aquatics and atmospherics."

"What about human chemistry?" she asked.

Okay, getting more specific.

Knowing exactly what she wanted and guessing she could get it, she demanded, "What about our chemistry."

That had his head bolting up, eyes wide, mouth agape. What had she meant by that? Did she realize the double entendres they'd been playing with ever since chemistry had come up? Of course she did, she wasn't brain dead. So did she realize what she'd just said? Had she done it on purpose? What was she aiming for?

"I mean, tell me what chemistry is the same for all aliens." Her clarification told him instantly she was doing this on purpose. She'd taken far too long, clearly engrossed in watching his reaction. She was having fun at his expense.

"I don't think there's anything that's the same for all aliens. And you're thinking biology, not chemistry." Best to escape this train of thought. Biology led to anatomy. Anatomy led to compatibility. Maybe he could escape back into the confines of a discussion about metal oxides or inert gases; they sounded a lot safer than 'their chemistry'.

But she wasn't having it. "We look the same, we'd be pretty similar wouldn't we?"

"Chemically and anatomically?" he queried. She nodded in response and he pondered for a moment how exactly to tell her that chemically and anatomically they were almost identical. In fact, the only differences were the slightly different brain structure and the number of hearts and kidneys, and that was just a kink in natural selection. One could argue you didn't need two hearts just as easily as argue that you didn't need two kidneys.

After several seconds thought and a mock glare from her that clearly stated, once again that ignoring her would not be acceptable, he forged on. "Chemically we're almost identical. There are a few interesting anatomical differences."

What? She had to bite her lip to stop from asking him for specifics. Why was she even interested in anatomical differences?

He reveled in the shocked look that overtook her and, what was that, disappointment? "Nothing seriously inhibiting though."

What did he mean by that? He winked and look downwards cheekily and she felt like going 'Ohhh…' and sidling on over but was stopped by the smug look he was sporting. Had he seen the glint in her eye?

He'd definitely got one over her. He hadn't even thought about it when he'd forayed into the anatomical disparity between them. She wasn't thinking about hearts and kidneys, she was thinking something more specific, more along the lines of compatibility.

"But the chemical senses are intriguingly identical." He said it sounding almost like he always did, managing to keep most of the laughter and curiosity out of his voice.

Rose looked at him and saw a fully-fledged self-satisfied smirk. God, he'd read her like a book, the 'anatomical differences' had probably been a line just to try to get her reaction. Point to him then.

"As species, I must admit, humans and Time Lords are very compatible."

A shiver ran, uninvited, up her spine. Bastard. Back to safer ground: "What are chemical senses?" Good: an intelligible sentence with structure, a point and she sounded like she was listening and taking notes.

"Taste and smell. Technically all five of the regular senses are attributed to chemical reactions but taste and smell are the only two that depend solely on one on one chemistry." This was better, back on track, she'd picked a specific area of chemistry to discover and he was communicating the ideas well.

"So we're the same in the way we taste and smell things?" Once he'd nodded the affirmative, she went on, "So you have that thing where you taste sweet on the tip of your tongue and sour at the back and all that."

Stupid little humans. That entire idea was just completely wrong. All the academics knew it was wrong. It had been proven wrong. And all before the twenty first century and yet most people still believed it. He told Rose so and then elaborated, "You see, it's actually more complicated than text books like to make out. There is a particular taste bud for each of the four tastes: sweet, sour, bitter and salty. You can see them if you poke your tongue in the mirror. Each of these tiny taste buds has lots of microscopic binding sites –"

He was watching her face the whole time, admiring her (only for her mind, of course) as she took it all in and tried to organize it logically. The moment he said 'binding sites,' he knew he'd gone too far, too fast, he could tell because a little crease appeared between her eyes and her tongue disappeared so she could purse her lips. He backtracked, "Binding sites are like little holes. They're specifically shaped so that only things with a certain structure will fit in. Think of it like a lock and key. The binding site is the lock; food, on the molecular level, is the key."

"So there's a different lock for each taste?" He nodded, though he looked hesitant at agreeing. "So does that mean that everything that tastes sweet has exactly the same key? There's a bit of molecule that is exactly the same for everything sweet?"

So intelligent; she'd caught on fast. Jumping to the conclusion that the human race had taken years to reach. Sadly, though a brilliant solution, evolution had been more eccentric in its design and now he had to try to teach her that. Still, he couldn't help but grin in delight, wanting desperately just to give her a hug and tell her how impressed he was. He'd be damned if he wouldn't have her synthesizing in the lab within the year.

"Well done," he told her, trying not to sound quite as astonished as he was. "That's the basic explanation but it's not as easy as you think." A pause to order his thoughts. "Salty is easiest. Any metal salt fits in the binding sight. There's no distinction between different kinds of salt, they all taste the same: salty."

"Hang on," she interrupted him. He'd bounded ahead without waiting to see if she was still with him. "What's a metal salt?"

"Oh," he traced his words back, "Sorry. Metal salts contain at least one metal atom and one group seven atom." She still looked lost. Maybe he needed to get her a Periodic Table. "Okay, well, sodium chloride is basic table salt. It's one atom of sodium, the metal, joined with one atom of chlorine, the group seven atom. Just trust me on this, any metal joined with chlorine will taste salty."

He watched her as she discretely lifted herself to sit perched on the edge of the console, hands next to her thighs, her hair tucked behind her ears as she continued to hang off his every word. "Sour's really easy. Anything that's an acid sets off that particular taste bud. To be an acid, a molecule has to have a hydrogen atom that can disconnect taking away a positive charge."

"Isn't that physics? Charge and all that?"

"No, everything can be traced back to chemistry." There was a tinge of anger and frustration there; he didn't like her bringing up physics. "Every single atom has the ability to become charged with electrical energy. It doesn't matter;" he said dismissively. "Just remember that acids are sour. Think lemon juice or vinegar."

She smiled, his excitement contagious, his eagerness to get to the next topic irrepressible.

"Bitter's difficult. There isn't really any pattern, each taste bud actually has a whole multitude of different locks, each for a different key. Too hard to try to explain. The taste that's ingenious, saved for last because it's the best," he flashed her a tempting smile just because he could, "is sweet."

"Right." She sounded cheeky; she wasn't even sure how that fit in with the conversation. Actually, that smile he'd sent her had been a little cheeky. "And why is it so fantastic then, Doctor?"

The fact that she'd used the word 'fantastic' and was giving him that completely enticing look that combined wisdom with innocence and shyness with audacity almost made his knees buckle. When had it become this? That she could reduce him to such uselessness with just a look. Ever since chemistry had come in to play. It was his own fault; he'd started it. Or was it her who'd first brought it up? His gaze, which had been sitting somewhere near the bare inch of skin that showed above the top of her jeans, happened to flicker up to her eyes. Ah, yes, talking. It was his turn to say something.

"It's so fantastic," he was happy to see her swallow as he put a little extra accent into the word, "because it's subtle."

Well, she'd died and gone to heaven. Was it even possible just to die from words? He was getting far too good at these games, far too bold. Staring at him, she could imagine the way he was thinking, counting up the probabilities of each of his words having a particular effect, formulating his next attack.

"Sweetness is detected by the tongue based on a very specific but easy to make, break and eat chemical triangle. It has a certain shape with lengths and angles set and at each apex there's a particular arrangement of atoms. One apex forms a hydrogen bond, one is water repellant and the last is basic, the opposite of acidic."

He stopped suddenly and she looked up, she was barely making sense of his words, but she was still interested. Fascinated by the way chemistry worked, the way it manipulated everything on a level that was invisible, the way it made him. "Is that it then?" Damn her chemistry, next time she was asking him what made her mouth go dry and her voice breathy whenever they had these conversations.

"Very specific but subtle. By themselves these characteristics mean nothing. Put them all together and you have the wonderful world of sugar, cake, candy and chocolate." She didn't fully grasp what he was saying, he could tell, not many people could have understood without a lot more background, but she did get the basics. Now, if only he could back away slowly and not have it end like their previous two chemically charged encounters had: with both breathless and having difficulty with rational thought.

A deep throated groan at the thought of chocolate escaped her lips before she could stop it "God, I haven't had chocolate in months."

Okay, so not backing away from the situation. It was a miracle he hadn't moved forward. In fact, he probably would have except for that annoying lack of connection between his brain and his legs. As was often the way around Rose.

"No chocolate?" Yep, he was completely breathless. He didn't even know a voice could be that husky. She wasn't stupid, she must be seeing through him like he was a window.

"None."

Instantly he realized her voice was identically husky, shaky in the same way, unsure yet insistent. Strange. That seemed to mean something.

"There's heaps in the kitchen."

"Not the nice kind. It's cheap."

When had he taken a step forward and why was she angling up? She'd slid off the console, three or four inches between them. Close but not touching.

"Ah." Triumph, even if he couldn't think straight and was actually clenching the insides of his pockets to keep from grabbing something else. "The good stuff is kept in my room." That came out wrong. He should not be mentioning his room.

The look that passed over her face was golden. Shock, anger, aggression, but all in play. It wasn't about the chocolate. They both knew that. "Why don't I get any?"

"You don't appreciate the chemistry." Fast, make it banter, if it's banter it's not sensual. That made sense, right?

"I do now. Give me some."

He had some in his pocket. He always had chocolate in his pocket. Three or four pieces wrapped in gold foil, made by hand in the Swiss Alps. The perfect chocolate. And she was right, the chemistry was there. He took it out, holding it just below his chin in front of her eyes.

"Say please?"

She gave him that look again, smiling through her eyes and curling her lips up, poking her tongue out and letting her eyes flick between him and the chocolate. Rose was not about to play his games. She knew what she wanted. Did she have the audacity to take it?

Raising a hand, she let her fingers encompass the hand that held her chocolate, her eyes refusing to leave his even though his flickered down twice. She let her thumb skirt over the back of his hand before taking the foil covered treat from him. Unwrapping it, breaking it in two, she devoured the first piece.

He didn't try to stop her. He couldn't. He was enchanted, watching her as she broke it in two, eating the first half quickly, just wanting to get at it. He knew full well that his mouth had dropped open. As she slowly savored the smell and taste of the second piece, letting it melt on her tongue, he knew he was so far past gone, nothing in space and time was going to pull him back.

Mouth dry again, he swallowed. She was staring at him and he knew he was staring back. She was smiling, he was gaping. Using every ounce of brain power available, he closed his mouth. She was not going to win another point, he decided. If only he had a plan, or some semblance of control.

She swallowed the last of the chocolate smiling at him and lifting a finger to wipe the corner of her mouth. They were still only inches apart. "So that was all just because of the chemistry?"

He had an opening; he could see it, taste it. Would he dare take it? Would he be able to stop himself from going too far? "All in the chemistry."

It appeared, to Rose at least, that she'd won this round. But now she had to get out, this was all beginning to overwhelm her. Her head felt fuzzy, her limbs were heavy but her senses were alight. She could feel the heat coming off him, the electricity between them. She could smell him, a surreal mixture of chemicals that changed daily, not overwhelming like one would expect; it was subtle, just like he said. She couldn't place today's smell, but it was there. Chemistry.

He grabbed her hand, the one still wavering in front of her face, a teasing gesture. She'd been about to lower it, leave and hide in her bedroom. But now, mentally and physically, he had a hold of her and who was she to argue.

"You're taking advantage of our chemistry." He sounded strong, in control, so far from what he actually was that at first he wasn't sure he was hearing right.

Rose just pondered over whether or not he realized what he was saying and if he knew about the tremors she could feel running through her spine.

"You missed a bit." Soft, but not exactly delicate. It was the first time he'd actually thought about what her hands felt like, now he was close enough to see the tiny scars, some of which he knew were his fault, others he couldn't identify causes for.

"What?" God, she sounded daft. What was he talking about?

"Here." He turned her hand in his own, and she saw, to her horror, a tiny speck of chocolate on her pointer finger. Must have been transferred when she wiped her lips.

Turning the hand back around to face him, he finally lost the battle, letting his eyes flicker from hers to where their hands met. When he looked back up, he was happy with the result. She looked shocked and terrified and hungry, all at once. He guided the hand towards him, closer and closer, covering the few inches over several seconds until it was bare millimeters from his lips.

He held his breath and noticed she held hers. The chocolate, tiny and dark, smelt incredible, or maybe that was Rose. He exhaled, loving the way she tensed as his breath hit her skin. Then he dropped the hand, letting it fall away as a dead weight, watching as it fell back to her side.

Grinning, he told her, "I'm getting some lunch. You want to come?" Okay, so he'd admit, that might have all been a little more convincing if his eyes weren't still locked with hers and instead of the husky aspect his voice had taken on, he'd sounded playful and innocent. But she was shocked. He'd won. And, he thought rather happily as he turned on his heal and all but ran to the kitchen, nothing had exploded.