That’s how he knew…

For a second after she suddenly snapped back to wakefulness, Twilight wasn't sure what had prompted it.
Blinking slowly, she shifted on her resting place and looked around the room, mind and vision still clouded by the deep sleep she had been awakened from. It was still night possibly and the fire was still burning, but there seemed to be another source of light in her face. Blinking a few more times, she realized that this new light coming through the window from the faint beams of sunlight hanging in the sky. So apparently the rain had stopped and the night was over, the dawn being visible on the horizon.

But none of that mattered as she easily remembered about how and why she was here, and her face burned as she felt herself smile.
The memories of the passion that had transpired the night before, on this very floor atop a blanket she had managed to pull from her magical inventory for them, flooded through her mind and made her feel a flock of giant butterflies in her belly trying to find their way out.

Rainbow Dash’s forelegs around her barrel as they held each other tightly.
Those beautiful wings embracing her as well and each feather acting like it’s own separate entity as they ran down her sides and flank.
The Pegasus’s lips on her own, eager, demanding, as they rolled on the floor in front of the roaring fire, taking turns being on top even as the more athletic pony wound up being more dominant and winning more often then not.

The most wonderful actions that took place over the course of the whole night, the taste and feel of the cyan pony, the glorious sounds she'd made and words she’d whispered as they lost themselves in each other.
The pure and utter happiness that Dash’s show of trust and declaration of love had given her.
The unspeakable joy that she had felt as Dash sent her careening over the edge into heavenly oblivion, their intimacy somehow becoming 10,000 times greater by what the Pegasus had given the Unicorn and banished any remotely possible fears and uncertainties of potential abandonment.
The few moments that followed before they slipped into slumber, warm and comfortable, fur pressed to fur, cyan muzzle to lavender cheek, tails and limbs intertwined.

And now this, recognizing the feeling of a kiss on that same cheek and realizing that it was the reason she had woken up after such an intimately exhausting night of love.
Twilight’s face heated up just a little, but then something drew her to look off to the left of her vision, and saw the mare who held her heart in her hooves sitting down and facing away, doing something that the Unicorn could not tell for sure.

Slowly, Twilight rose to her hooves as carefully as she could, biting her lip as she felt her legs wobble just a bit before steadily regaining their overall functionality. She made sure to first be sure that she could stand and walk and would not fall over or anything, and then slowly walked across the room.
She saw one of Dash’s ears twitch two times slightly, an obvious sign that she heard Twilight approaching, but the Pegasus didn’t seem to move much at all, her head still downturned to a minor degree.
Taking a step to the left side to try seeing if she could find out what it was that was taking up Rainbow’s attention so curiously, Twilight could see partially around the left side of Rainbow and realized that she was actually reading a book:
The latest Daring Do book that Twilight had brought here with them and that was the very first copy that had been printed!

Why that little schemer!

“Oh so you’re reading my book before me on the sly, eh?”
She asked with a chuckle. She saw Rainbow Dash twitch once, and the very apparent sound of a barely-concealed laugh reached her own sensitive ears.
Twilight smiled and shook her head, then closed the distance and out a hoof on one of Dash’s shoulders, pulling it slightly, and making the cyan pony turn her head with a slight smile and a twinkle in her beautiful eyes.
The lavender Unicorn didn’t say a word. She simply leaned in forward and pressed her lips to those of her beloved, moving slowly and sensually. The latter’s left foreleg slowly rose up and her hoof pressed against Twilight’s head as she gently stroked the Unicorn’s mane, the book gently laid down on the floor in front of her for now as they continued to move their mouths on each other’s.
They would need to return home soon enough today, and so were using what they could of their last minutes and moments of intimacy together alone here, because they would not be able to do any of that anymore once they were back in Ponyville and with their friends and doing their jobs.

Twilight felt it again for the thousandth time as they kissed. That wonderful feeling of warm comfort coursing through her body at the touch of the Pegasus who had somehow managed to take hold of her in a way that was almost impossible to ever see happening to herself.
She loved this mare, everything about her. The way she felt, smelled, tasted. The once infinitely-studious pupil of Princess Celestia had found herself something and someone who could very well challenge her addiction to simply reading and keeping checklists. Rainbow Dash had shown her how truly fun and enjoyable it could be to simply stop thinking about the future and start living in the moment. How much worth there really was in just taking it one day at a time, and that being able to enjoy the simple things in life were just as important as other more complex ones.
Simple things like taking the time to meet with the one you love, and spending time with them in ways that you honestly cannot with anypony else.

But it wasn't just the physical touch and sense of exciting freedom that made everything so very worth it to Twilight, not at all, FAR from it!
It was also the way Rainbow felt pressed against her breast as she held the athlete from behind in a half-hug.

It was the way her hoof tenderly caressed Twilight’s mane as they softly moaned into each other’s mouths and the kiss got just a little more intimate, gently stroking downwards through the smooth hairs with a soft touch that nopony would have believed the competitive Pegasus to be capable of.

It was the way that she felt safer and happier from just a simple glance at any given moment on a random day, and so much more than that when having precious HOURS to spend together alone with her, like right here and now.

It was the way Rainbow would specifically reserve a special smile for her, a special level of touch (like the current mane-stroking) for her, a special way of speaking for her, a special look in her eyes… all for HER. All of this special tenderness and unmatched kindness, given freely by the greatest flyer in all of Equestria who’s sense of loyalty and duty would only end in death… to Ponyville’s resident book-loving introvert, who at one point had never entertained the idea of romantic love for herself but now had to wonder just how she had ever gotten along without it.

Even now, with her mind clouded by all these amazing sensations, all of Twilight’s attention on what they were doing shortly before they would need to leave this special place and head back, she couldn't help but think about how much she loved this special, super competitive fast-flyer. How she made life so much more beautiful and precious better just by being in it.
The way she'd supported the poor over-worried Unicorn ever since they’d first met and embarked on that first mission to defeat Nightmare Moon.
The way she had her back at every turn, fought at her side without hesitation or even asking for a reason why or anything like that.
The way she could be so brusk and tough about a great many things, only to then turn around and become the most attentive and caring soul Twilight had ever met.

The simple things, the little moments they'd spent together, eating, relaxing, just talking. The unbelievable depth of understanding she'd shown for Twilight, for who the Unicorn was, the way she was, the choices she had made.
Rainbow knew her, knew her better than she'd probably ever even been known before(with the probable exception of dear little Spike), and the knowledge that this brave, amazing, talented, wonderful, and totally AWESOME Pegasus wanted Twilight just as much as Twilight wanted her, in every possible way… was nothing short of absolutely incredible. Purely and utterly humbling.

One night. It was more often than not just one night they’d be able to have at a time.
One night to be together, give each other what they'd been so desperately wanting. Needing. Missing.
But it always turned into something so much more special and fulfilling. It felt as if for every time they managed to meet up, they would love a lifetime’s worth.
It was so much more than just physical enjoyment. What they had… Twilight could only best explain it by how she felt about Rainbow Dash herself:

It was everything.

Rainbow Dash… was everything.

Lost in her thoughts, it took Twilight a second to realise when Rainbow had made the kiss turn Prench, named after the country of Prance which was famous for, besides many other things, producing the supermodel Fleur Dis Lee and the highly romantic Prench Kiss.
Her mind nearly short circuited by this turn of events, and increasingly breathed harder through her nostrils as her heart beat ever faster.
As if sensing her turmoil, Rainbow eased the pressure on Twilight’s mouth and pulled back, both their breaths mixing again, before she silently asked, “You OK, Twi?”

It was like she wasn’t really expecting a response. And it was a response that Twilight didn't have anyway, because she had something more, something far better on her mind right now. Something she desperately needed to say.

Slowly, she walked around to face Rainbow and reached up, gently taking her face in her hooves.
"I love you, Rainbow Dash."
The face of the beautiful cyan Pegasus softened even further now with Twilight’s fervent declaration as the Unicorn gently stroked her cheeks with her lavender hooves, her voice soft and yet thick with pure emotion.
“I love you so, so much.”

“Twi, I… I love you so much more than I can ever say.”

Silence fell between them as they gazed into each other's eyes, neither wavering, completely open and vulnerable, both seeing exactly what they were. What they meant to the other.
It was yet another moment Twilight Sparkle would never forget, staring at her beloved Rainbow here and now, knowing that she was looking at her future.

Twilight smiled as tears formed in her eyes, and pressed her forehead to Rainbows. They both just sat there like that.
Sure, they would need to get back home soon and go back to their everyday lives, but for now, they just sat together in peaceful silence…

And then Rainbow kissed her again.