Warnings: Okay, so this is the first time in pretty much ever that I'm not too worried about keeping the characters where they belong. See, this little thing popped into my head and wouldn't leave until I made it leave by writing it. That said, there's definitely an OOC Kate (though I tried to explain why I have her reacting the way she is), but for the purposes of this fic, we're going to suspend our disbelief please and maybe laugh at the humour of the whole situation.

This isn't meant as any sort of deep exploration of character. I have a couple of those on the go right now, both posted and in the process of being written. This was just meant as a fun fluff piece with no real point and probably no continuation even though there's theoretically room for one.

So! Now that I've put that information forward first, let's get to the fic! Prepare your toothbrushes, please! I'm pretty sure I have at least one new cavity from writing this. Well, either that or I've lost some brain cells from a rather fantastic mind-altering substance. I'll let you readers be the judge.

Oh, and this takes place after "The Late Shaft". So the rest of the season technically hasn't happened.

Chemistry Lesson

Kate looked up at the knock on her door. Her brow furrowed. It was late and she definitely wasn't expecting company. It made the person on the other side of the door even more surprising because though Rick Castle wasn't the type to necessarily barge in, he had a habit of calling her before finding her outside of the precinct or a crime scene. But that didn't mean she wasn't quick about throwing the locks and sliding the chain to the side to open the door.

"Castle? What are you doing here?"

"I need your help," he answered, looking at her expectantly.

Her brow furrowed, but she stepped back. Her help? She took him in carefully. He seemed anxious, nervous almost, so she assumed it wasn't about Nikki Heat. If it was about her fictional alter-ego, he, a) would have called and b) definitely wouldn't be nervous about it. While she ragged on him about Nikki's slutty behaviours, she was actually kind of happy with the way the character turned out.

And if Alexis was in trouble, she was sure he would be more frantic about it. He probably would have called her in that panic, rather than just showing up. So it was either an epiphany or a realization – which kind of made her heart speed up in her chest – or there was something he needed to talk out. Maybe… no, it couldn't be Ellie. That wouldn't make sense. She'd seen the look on Rick's face when he discovered Ellie Monroe had slept with the TV station owner to get a part and she severely doubted he'd go back to that. So this had to be something else. Still… She had to be sure.

"Castle? Is this about Ellie Monroe?"

He spun to face her from his pacing. "No," he said, eyes wide. "No, she's… gone."

"I'm sorry," she replied on a sigh and really, she was. While she was only trying to look out for Rick, that didn't mean she wasn't upset that it hadn't worked out. They were friends, they wanted what was best for each other, and if that meant a girl in his life, she wasn't going to stand in his way.

It didn't matter that he made her stomach flutter.

It didn't matter that he made her days lighter.

It didn't matter that for the few weeks it had taken her to find a new apartment, she'd felt at home at his.

It didn't matter that his family treated her like one of their own.

No, what mattered was Rick and if he wanted someone like Ellie, then she wasn't going to stand in his way. Plus, it wasn't like she had feelings for him. They were friends, but that was it.

"Sorry? Why?" But he waved his hand when she opened her mouth to answer. "Never mind."

She felt a little hurt at how quickly he'd brushed her off, but then his eyes locked on hers. His blues were intense, probing, and part of Kate couldn't help but feel naked at the perusal. His gaze didn't wander from her face, but she still felt vulnerable. "Do you believe in chemistry?"

"Like atoms, molecules and chemicals?" she asked in confusion.

"No," he replied. "Chemistry between people. Sexual chemistry."

She had it on good authority – and she considered Lanie good authority – that sexual chemistry was alive and well, and most especially in the precinct. She could think of a handful of partners that it rang true for and, admittedly, that included her and Rick. But chemistry didn't mean relationships and she wasn't going to be a notch on Rick Castle's bedpost. And she wasn't about to make him a notch on hers. There was too much at stake. Their friendship, their working relationship, Alexis… And yes, she considered Alexis a factor in making a decision like that. They'd become close during Kate's stay and she wasn't willing to put that at risk. Even for a night she was sure would be mind-blowing.

"Yes," she finally said. "I think sexual chemistry exists."

"Okay, would you say sexual chemistry makes physical affection, say a kiss, more potent?"

Kate arched an eyebrow. "Yes," she agreed again. "Castle, where is this coming from?"

"I was talking to Alexis," he replied, and waved her off again as if the point was irrelevant.

She huffed out a breath and folded her arms. "I'm not answering any more questions until you tell me what the hell is going on."

He looked up at her, a little surprised that she'd pulled out the interrogation voice. "What?"

"You show up at my apartment late at night talking gibberish about Alexis and sexual chemistry." She couldn't help but feel a thrill of victory when he winced at 'Alexis' and 'sexual chemistry' in the same sentence. "And you're not giving me a clue as to where this is going, how this links to why you need my help and what the heck brought all this on to begin with."

Rick took her in and she knew she had her best intimidation glare plastered on her face, looking to all the world like she was her usual Detective Beckett. She felt confusion well up when he took her in and grinned.

"You know, you look much less intimidating standing there in Winnie the Pooh pajamas."

Kate rolled her eyes as she looked down at herself, taking in the blue flannel pants and yellow tank to she wore. "Come on, Castle. Give me an explanation."

"Alexis came home just as Ellie was leaving and she jumped to all the right conclusions… anyway, she gave me a lecture on starlets and strange women and somehow we segued into sexual chemistry."

"Wait, so you can talk about your sexual chemistry with your daughter but the minute I put Alexis and sexual chemistry within three words of each other you wince?" She almost laughed when he did, in fact, flinch at her words.

"That's my daughter."

"And you're her father! You can't see how that would be a little disturbing?"

"I know! But she's like a dog with a bone and she wouldn't let it go!"

Kate sighed. "Go on."

"Well, you know Alexis is a little bit of a romantic." It was an understatement, but Kate nodded anyway. "Well, she told me that while sexual chemistry was probably great, it would probably better if the woman was someone I had feelings for."

The butterflies started up in her stomach, but Kate resolutely ignored them and focused on the facts while Rick focused on pacing her carpet again. "And you said…?"

"Well, as much as I can be a romantic myself, I like to think that I've seen both sides of the coin. Not often mind you, but enough to know the difference."

She swallowed. She was aware of his reputation. Very aware of it. "And?"

"And she challenged me to prove it," Rick replied coming to a stop right in front of her.

"Okay," Kate answered, forcing her breathing to remain slow and normal. "I don't get how I can help in this."

The nerves were back and he was almost wringing his hands in front of him. She almost laughed when he shoved them into the pockets of his coat, but the idea that he was so intensely nervous had her a bit on edge. He was about to say or do something she didn't totally agree with and she was torn between whether he was just going to blurt it out, or whether he was going to beat around the bush and make her guess. Either way, she wasn't totally sure about what would happen.

"Look, Kate," he began.

Her stomach jolted from the use of her first name. Oh, she definitely wasn't going to like this.

"You trust me, right?"

Sometimes true, sometimes not, but when it came down to the nitty gritty of it… "Yes."

"Okay, so you know that I'm not going to hurt you, right?"

She assumed physically because he'd already hurt her emotionally once. "Okay…"

"And I'm only doing this as an experiment, to prove that feelings don't change sexual chemistry."

Uh oh. Her brain clicked and her eyes widened, but he'd already stepped towards her and grasped her shoulders. His lips were on hers before she had the chance to effectively protest.

If she was honest, she'd thought of this moment probably too many times to count. There was no doubt in any one's mind that there was sexual chemistry between them. The banter, the flirting, it proved it. But she'd always figured that when he kissed her, she would have the presence of mind to shove him back and slap him. But that wasn't her body's initial response to the action. Instead of recoiling, she sunk, utterly and completely, her mind blanking and her body heating almost instantly. She was kissing him back before she even had time to register that they were kissing and even when she did, she didn't stop. Her hands came up to his biceps as his curled around her waist and she heard a growl – whether it was him or her, she'd never be able to tell – break the silence of her apartment.

He was the one to ease away, the one with the presence of mind, and her fingers clenched on his arms as she came back to herself with a thump. Even so, she didn't slap him, just stood there, still a little stunned. He was breathing heavily as he looked down at her warily and she dimly realized she was supposed to react. Which meant she had two choices: slap him now, which seemed largely useless since the kiss was already over and her flabbergasted look was probably enough for him to gather she actually kind of liked kissing him, or act as if it wasn't a big deal.

Option B, Alex, Kate thought to herself and slowly took a deep breath. "So?"

"So what?"

Oh, it was nice to see he was as stunned as she was. And kind of flattering. "What's the conclusion?"

"What?"

"Did you fail ninth grade science? You did an experiment, you had a hypothesis, what's the conclusion?"

He blinked, seemingly trying to get himself back under control. "Oh," he said, then paused, seemingly considering before his face broke out in a wide grin. "I was wrong. Alexis was right. I should have known she would be, even though she's a teenager and I really, really, don't want to know how she figured it out." He headed for the door.

That was it? That couldn't be it. They'd just shared a mind-blowing kiss in the middle of her living room and he was just going to walk away? He hadn't even told her which side she stood on! Yet the other half of her told her to leave it, drop it, because talking about the kiss, about what it meant, could lead her down a road she either didn't want to go down or wasn't ready for.

"Wait," Kate blurted and slammed her eyes closed. She didn't want to know. She didn't want to ask the question. Her arms wrapped around her stomach. If she didn't want to know, why had she told him to stop?

She heard him step back toward her. "Yes?"

She turned, forcing her eyes open and a smile to tilt her mouth. "Never mind, don't worry about it."

But his smile was already curving up into a smirk and she wanted to curse, but didn't want to give him the satisfaction of knowing the kiss had gotten under her skin. "Oh, no. Now I really want to know what you were going to ask. Because, it's quite obvious, my dear detective, that you do have a question on the tip of your tongue."

It was a bad idea. It was a really bad idea and she didn't do vulnerable little girl, but he'd kissed her and it was in the context of an experiment and, really he'd done it without permission so he owed her an explanation… She sucked in a deep breath. "Which one was I?"

"I beg your pardon?"

Oh, as soon as she got over the shock of everything that was going on, she was going to make him pay. She sunk her teeth into her lower lip, trying to find the words.

"Oh! You mean were you the just-plain-sexual-chemistry or were you the sexual-chemistry-plus-feelings."

Only then did Kate meet his gaze, searching them. She found playfulness there, but also a surprising sense of affection and peace. Pride, definitely, for kissing her and not losing a limb, but she didn't feel like he was mocking her, making fun of her. She had the sense that he wasn't going to hold it over her head. So she just waited for his answer, her fingers gripping the skin of her waist tightly, protectively.

"I'm surprised you even have to ask," he finally said, stepping closer again, retracing his steps until she had to tilt her head up because of their natural height difference. He leaned down, the same way he had when he'd requested she never, ever call him 'Kitten', his mouth and his breath brushing her ear. "Feelings, of course."

Then he sauntered out of her apartment.

She stood, a little stunned, a little taken aback as the door closed and finally let out a guffaw of laughter. Good Lord, the man certainly knew how to keep her on her toes. Yet, there was a part of her that trusted his admission, that saw the truth in what he said. But it made another point abundantly clear to her. He knew she wasn't ready, for him, for a relationship, who knew, but either way, he knew. Yet he'd pretty much told her that it was okay, that he'd wait for her to be ready because whatever they had went beyond sexual chemistry.

Kate grinned as she made her way to the door, putting the locks and the chain back in place before heading to bed. She was still going to get him back sometime in the near future, but there was a part of her that felt better because she knew that someday, when she was ready, when they could prove to each other that it was a long-term thing and that their fears were unfounded, they'd see if a relationship would really work.

After all, even Kate knew that feelings and sexual chemistry were the best foundations of a relationship.