A/N: This is a Valentine's gift to whatifellinlovewith on Tumblr, as organised by caskettvalentines.

Also, apologies for slightly altering the first line of the poem by Elizabeth Barrett Browning.

How do I know thee? Let me count the ways

It's the less obvious things that he finds intimate, that fascinate him: the crease of her inner elbow from which he can trace the blue veins down to her wrist where he can see her pulse thrumming next to her radius; the hairs at the base of her skull, soft against his seeking fingers; the sweep of her collar bone over to the rounded smoothness of her humerus nestled within her scapula. And this is where his attention is currently focused. Her comfortable top has slipped a little (perhaps with a little of his encouragement) and he's completing his own private game of dot to dot over the freckles dusting her shoulder.

"No one has ever explored me like you," she murmurs into the silence of the night.

"You're the greatest mystery I want to solve."

She snorts at his romanticism, but counters, "Let me do the same for you."

"Do what?"

"Let me know you, Rick."

He frowns, feeling the comfortable atmosphere ebb away at the cryptic words. "You know me."

She shifts, dislodging his hand. "Do I?"

His confusion deepens, certain that he's missing a vital piece of information that once revealed will clarify her meaning. "Where's this coming from, Kate?"

She sighs and sits up. He follows suit. His body feels the loss of her warmth keenly. With them both sitting, and with a topic that seems far too serious for such a late hour, the distance between them seems so much vaster than it is. With no warning, he hauls her across and into his lap, paying no heed to the sheets bunching and folding around them. Still, Kate settles against him. She spends a few quiet moments untangling herself and smoothing the brushed cotton bedding over them. He lets her; whereas he has a tendency to blurt out whatever he is thinking, his partner struggles with being open and likely needs a little time to gather her thoughts. So he is content to feel her warmth against him once more while he waits her out.

He feels her sigh more than hears it, through the expansion of her ribcage pushing against his. "Meredith… I asked her why the two of you didn't work out."

Well that's made everything no clearer; he's already told Kate the story behind the demise of that particular marriage. "But you know she had an affair. That pretty much signalled the end of our relationship."

"No, I know. But why did she do it? Why did she risk everything?"

Surely she can't be insinuating… "You mean, whether I pushed her into the arms of another man?"

"No!" she gasps out before he's even finished speaking; horror inherent in her tone, her expression and the sudden grip on his bicep. "Rick, no, of course not. You're this amazing guy and I couldn't understand why she'd be willing to give you up."

He can't help the warmth that her words provoke, even if he is still nervous at whatever his ex-wife's response was, and why his girlfriend had such a negative reaction.

"I'm sorry, Rick. It's private and I shouldn't have asked her."

He shrugs, "You were curious. I'd have wanted to know."

She raises her eyebrows as if to tell him his words are pretty obvious; history is enough evidence of this. Then she shakes her head. "I still should have kept quiet. Or asked you instead."

That raises a point. "Why haven't you asked me before now?"

It's Kate's turn to shrug, her arms shifting against his solid form. "It never really occurred to me, aside from the occasional bout of wondering, I suppose. But confronted with your ex-wife these past few days… I don't know, Castle. There were little things like how she made your coffee. It's obvious you had some good times."

"Kate, you can't think-"

"No, I know," she interrupts again. "I know you will never cheat on me. I guess… I guess you were right; I was just curious."

The way she's avoiding his gaze, and obviously itching to be out of his embrace and flee from the discussion, he knows something else is going on. He says as much to her, gently, not demanding, his fingers sliding soothingly over her lower arm. "Kate?" he prompts gently when she remains contemplative.

"I guess," she starts. And now her eyes are downcast and focussed on a loose thread in their bedding, her fingers picking at the cotton absentmindedly. Eventually she continues, her voice halting and unsure, so far from the Detective Beckett who'd near enough roared at a suspect earlier today. "If I knew the reason… If it was something Meredith did or didn't do…. I would know."

"Know what, Kate?"

She raises her soulful eyes to his. "Know how not to lose you. And I don't mean having an affair; I would never do that to you. I mean… I- I don't know," she trails off miserably, breaking their connection and making to move from his arms. He tightens his grip, however, bringing her back solidly against him.

"Tell me, Kate. Please," he prompts, only once she's deigned to curl up tightly against him.

She chews her lip; a characteristic gesture of uncertainty and seems to shrink a little more in his unyielding embrace. "If I knew why your relationship reached the point where she felt that risking your marriage was acceptable, then I would have some idea of how to stop that happening for us. I don't want to lose you, Castle. And sometimes I can't help thinking it's too good to be true. That one day I'll wake up to find it's fallen apart."

At the utter desolation on her features, his first instinct is to wrap her up tight and distract her with comfort and words of love and reassurance. But whilst that might be enough to placate her on the short-term, he knows that eventually her brilliant mind will continue churning over every last bit of their relationship. So he eases her back from him, strokes her hair behind her ear and drops his hand to lace his fingers through hers in a hold which he hopes conveys both comfort and support. "What did Meredith say?"

She's silent once more, and again he waits her out, until she resolutely holds his gaze. It seems she's thinking the same as he; this is a pivotal moment in their relationship and in order to move forward they need to release their old habit of allowing subtext to solve their problems.

"She said she knew nothing about you," speaks Beckett, immediately putting into context her words which sparked the conversation. "She said you know so much about her, and she knew comparatively little about you. And I think about us and I'm scared it's true." She blinks furiously back the tears born of worry, determined not to allow them to be a distraction. "You've literally written books based on me. And what have I done? What I know about you compares so poorly against that."

He's disagrees entirely with her words, but bites back his immediate refutation. Knowing how intrinsically being a detective is ingrained within her, she needs proof. So he provides just that.

He points to a scar on his elbow. "How did I get this, Beckett?"

"You were eight years old," she begins without hesitation, immediately realising his plan. "Martha was playing Lady Macbeth. You decided to dispel the superstition of speaking the name of The Scottish Play by running around the theatre screaming it. Soon enough you fell off the stage, breaking your arm which then required surgery. To this day you feel guilty that Martha gave up her role to stay with you through your recovery. But the memory is also a happy one for you as she also proved just how much she loves you, even though your childhood was unsettled."

The level of detail is astonishing, not least because he has never uttered a word to her about his conflicting emotions about the memory. Her perception is remarkable and he thinks that this on its own shows how much she knows him. Nevertheless, Kate didn't become the youngest female detective in the NYPD for relying on one paltry piece of evidence.

So they continue mapping out places on his body. Kate narrates each story like a pro, and Castle feels a swell of pride when her word patterns echo back to his story-telling-theorising at the precinct.

Eventually, he places her hand over his heart, his pulse quickening a little at the feeling of her pianist's fingers fluttering over his chest.

"You know what's most important to me."

"Alexis. Martha. Your words. Solving the story and bringing people the closure they need."

"And?" He jostles her when she fails to respond, attempting to coax her from her new hiding place against his shoulder. "And what else, Beckett?"

There's a moment of silence before a meek 'me' is uttered against his skin.

He squeezes her closer for a moment; he can't stop himself. Not that he ever forgets how important Kate is, but their conversation has brought the sheer intensity of his love for her to the front of his thoughts. He verbalises as much to her before continuing on his task of convincing her. "Kate, just because you've inspired me to write books, doesn't mean I know everything there is to know about you. There's still so much I want to learn, just as you have so much to learn about me. And now we're in this relationship together, we have an amazing opportunity to explore each other every day." He pauses to allow her to take in his words, to process the past few minutes. When she relaxes against him, he finds relief. "So we're good?" murmurs Castle eventually into her cherry scented locks, sifting his nose through the strands in a re-exploration of her being. The nod with which she replies shifts her hair against his chin and he presses a firm kiss to her crown. Suddenly, though, she pulls back, twisting to face him and gripping his shoulders.

"But we can't stop, Castle. I don't want us to stop finding each other interesting and exciting. Even when we've been together for-" she cuts herself off, clearly not ready to enter that particular minefield just yet. "Even when this relationship isn't so new, we need to keep exploring each other. We need to promise that to each other."

Her expression is so adorably earnest that he can't help but press his lips against hers, each of them whispering their vows in a mingling of breath.

Fin

A/N: Hope you enjoyed it, whatifellinlovewith! :-)