AN/Warning: Based on a post I saw on tumblr. Someone said they headcanoned Korra learning about engineering because it would make Asami happy. I took that cuteness and turned it smutty. Enjoy. The only warnings necessary on this piece are for explicit sexual contact between two adult women. Nothin' too fancy.

. . .

The Principles of Engineering

. . .

"Korra?"

The sound of her name made Korra sit up in surprise. She gasped, slamming the book in her lap shut. "Asami! Sorry, you scared me. I was… I was just…" She blinked, trying to clear the blurriness from her eyes. She had been about to say 'reading', but surely Asami already knew better.

"Sleeping," Asami finished for her instead. Firm palms trailed down from her shoulders, stroking her bare arms. Korra shuddered. Despite the warmth of the fire and the cozy armchair she had curled up in, the heat of Asami's hands felt wonderful. "Don't try to deny it. You were snoring over that book when I came in."

Korra gave her a sheepish look. "Maybe," she said, stifling a yawn. Her head was still thick, swimming with sleepiness, but the longer Asami touched her, the more awake she felt. Currents of electricity seemed to dance over her skin wherever Asami's fingers brushed.

"What were you reading, anyway? Something boring?"

That woke Korra up. She glanced down nervously into her lap to make sure the book was closed. "Not boring," she hedged, flipping it over so the title wouldn't show. "I was tired. Busy day yesterday."

Asami laughed and pressed a kiss to the top of her head. "A busy day lounging by the pool, if I remember. I'm sure you're exhausted."

A wrinkle formed in the middle of Korra's brow. "Hey, we just saved the entire world a few weeks ago!" She stuck her lower lip out in a pout, jutting her chin forward. "I'm allowed to be tired."

Asami ignored her. Instead, she continued staring down at the partially-hidden book. "You're avoiding my question. What were you reading?" She reached down, trying to fold her fingers around the book's spine, but Korra shifted sideways to block her.

"It's nothing. You, uh - you probably wouldn't like it..."

But Asami was too quick for her. She snatched the book and left the side of the chair. Korra mourned the loss of her lover's warm hands, but knew it was too late. There would be no hiding now.

"Iknik Blackstone Varrick's Guide to Awesome Engineering For Awesome People?" Asami laughed as she read the title aloud, shaking her head in disbelief. "Where does he come up with this stuff? And look at this subtitle: Learn how to master any machine from the Master himself. I can't believe his publisher let him get away with that one."

"It's actually pretty interesting," Korra said, trying to hide her embarrassment. "Well, aside from the parts where he goes on and on about himself and all the things he's invented. And, you know, the parts where he uses a bunch of big words no normal person should be expected to understand."

But Asami wasn't listening to her. She was already flipping through the pages of the book, skimming it with a mixture of interest and amusement. "Are all these notes yours?" she asked, flipping the book around to display some highlighted sections. "You've scribbled all over the first couple of chapters."

She gave a small shrug and reached back to rub her neck. "Well, yeah."

Asami suddenly seemed to notice her insecurity. She closed the book and set it down on the coffee table, perching on the arm of the chair. "Why?"

"How else am I supposed to keep up with you? Being the Avatar is impressive and all when there's a threat to the balance of the universe, but once that's over, what have I got? Not much. I guess..." Korra chewed on her lower lip. There weren't any good explanations aside from the truth, as reluctant as she was to share it. "I guess I want to make sure you don't get bored of me."

Asami stared at her in utter disbelief. Korra could see the shock on her face, and almost felt bad about voicing her insecurities until a gentle hand cupped her chin. When she tilted her face up to gaze into Asami's eyes, she forgot how to breathe. "Korra, I could never get bored of you. If three years apart wasn't enough to make my heart forget you, nothing could." Korra remained silent, unsure what to say, but Asami wasn't finished. "I never would have guessed you felt that way. You always seem so confident. So... I don't know. Cocky? Is that rude to say? It's charming, if that means anything."

At last, Korra allowed herself to smile. "It's the truth," she admitted. "I am cocky. Except when it comes to you. For some reason, I keep thinking stupid thoughts. Like that you'll change your mind and go after someone who understands the same things you do now that everything is over."

"That's not going to happen." Asami let go of her chin, abandoning the arm of the chair and sliding into her lap instead. "You're endlessly fascinating to me, Korra, even if you don't understand the..." She picked the book up again and flipped it open to the page with the most notes. "...the basic principles of mechanical engineering. Although you're pretty smart, too. I'm sure you would make a great engineer if you put your mind to it."

"I don't think so," Korra laughed. She was already feeling better, although that probably had something to do with beautiful woman hovering above her. Feeling Asami so close made her blood pound, and most of it was already starting to rush between her legs. She shifted beneath Asami's weight, trying to ignore the low, familiar ache in her lower belly. "I didn't understand most of what Varrick was talking about in that book."

Asami shut the book again and tossed it aside, not even bothering to watch where it landed. "You don't need to read Varrick's book to understand engineering, Korra. You've got Republic City's greatest engineer as your personal tutor. That is, if you want her..."

"Oh, yeah?" Korra couldn't resist sliding one of her hands around Asami's hip. A soft sigh escaped her chest as she found a comfortable place to grip. "Is she pretty?"

"That shouldn't matter," Asami said. "She's smart and good at her job."

Korra's smile grew wider. "Of course she is. I never said she wasn't. But, still... a girl's gotta ask."

"Fine," Asami huffed, although Korra could tell she was only pretending to be annoyed. "Yes, she's pretty. Gorgeous, in fact. And you're very lucky, because you're scheduled for your first lesson tonight."

"A little private tutoring?" Korra hoped her voice wouldn't shake as Asami's fingers began toying with the short hairs at the back of her neck. Her lips started drawing closer, and it took an effort of will not to surge up and kiss them. She looked further down, but it only made the situation worse when she noticed how close her face was to Asami's neck. It would be so easy to dip her head and taste that pale, unbroken skin...

"If you want. I bet I could teach you more than Varrick's book with a ten minute, hands-on lesson."

"Hands-on, huh?" Korra's eyebrows lifted in challenge. She had no doubt Asami was telling the truth, but she wanted to see for herself. "I like the sound of that. If you can teach me more than the book, I'll make sure to tell Varrick you're a better engineer than he is."

"I wouldn't say that," Asami said with false modesty. Her voice lowered to a purr, and the sound sent a pulse of want racing down the column of Korra's spine. "We have different disciplines. But I know for a fact I'm a better teacher."

Even though she was more than eager for the kind of 'lesson' Asami had in mind, Korra couldn't resist teasing her a little first. "You haven't had me for a student," she said, slipping her fingertips beneath the edge of Asami's shirt to skim the bare skin of her back. "Ask Tenzin. I'm not easy to handle."

"I know how to handle you."

Before Korra could speak, Asami pinned her to the back of the chair and sealed their mouths together in a blazing kiss. All the breath rushed out of her lungs, but she didn't care. Asami's lips were too sweet to pull away from. They kissed until her lungs burned and she sank back into the cushions, twitching all over with pleasant little shudders. At last, the heat pulled away. Korra noticed that her shoulders were still pressed against the back of the chair, pinned beneath both of Asami's hands.

"That was lesson one," Asami whispered, inches away from her mouth. "Force. A push or pull on an object."

"Force," Korra repeated, still in a daze. If this was the kind of teaching Asami planned on doing, she doubted she would survive the final exam. "I-it's not going to be easy to remember if you keep kissing me between every new term..."

Instead of taking her lips again, Asami's mouth wandered along the line of her jaw, finding a tender spot beneath her ear. Korra hissed as teeth grazed her skin. "Next term," she said, breath tickling her neck. "Pressure." One of the hands on her shoulder slid sideways, reaching around to unfasten the back of her top. Korra wasn't sure how Asami's fingers could move so dexterously when she wasn't even looking, but moments later, the front of her shirt fell open. "Pressure is the result of force being applied to a specific area..."

Korra couldn't quite swallow down her whimper as Asami brushed her shirt out of the way. Her hips jerked forward on instinct as a warm palm cupped her breast. "Um... I - I, uh... Pressure. Right. The result of..." She had no idea what she was saying, but she didn't have the clarity of mind to care. All she could see when she looked up were Asami's eyes. Her smile. All she could feel were Asami's hands. Her warm breath.

"One more. Friction." The soft kneading turned sharp as Asami's nails scraped over her skin, dragging to the center of her chest and scratching down her stomach. It didn't hurt, but the tingle did make her flesh light up with pleasure. "The resistance to motion when one body moves over another. It's the reason touch feels so good. And the reason we don't slip every time we take a step, but..." Korra let out a sharp cry as Asami's tongue trailed down along her throat, painting a hot trail to her collarbone. "You don't really care about that right now."

This time, Korra couldn't even manage to repeat the vocabulary word. She shuddered as Asami's hand wandered lower, lower, down to the edge of her leggings as that wicked mouth continued traveling everywhere at once. One moment, it was whispering into her skin, sliding down her chest. The next, it was latched onto her nipple, teasing the hard bud with the slightest hint of teeth. As much as she hated to admit it, Asami knew precisely how to undo her with a few simple touches.

"Of course, the properties of friction change depending on the surfaces moving against each other." Korra's eyes screwed shut as Asami's fingertips slid beneath the band of her leggings. They began pressing, rolling, stroking through the damp fabric of her underwear. Her hips bucked forward, but this time, they found purchase. A scream burned in her throat, but she swallowed it down at the last moment, biting hard at her lower lip. "If the surface is... lubricated... and warm..."

"Oh, Asami," Korra whispered, shuddering and tossing her head against the back of her chair as Asami slid beneath the last barrier. Her fingers stopped abruptly, and Korra opened her bleary eyes. "Wh-what are you doing?"

Asami laughed, leaning down for another kiss. This one was slower, but just as hot and thick, and it left her panting when it broke off. "Making sure you're still paying attention. As I was saying, if the surface is warm..." Her fingers started moving again, curling, sliding, exploring. Korra's breath hitched as they skated over the stiff bud of her clit, teasing it beneath its hood. "And wet..." They dragged further down, swirling around her entrance, dipping forward without offering any true penetration. "The friction decreases considerably."

Korra swallowed, struggling to find her voice despite the pleasant distraction between her legs. "I don't think this is in Varrick's book," she gasped, clutching tight to the arms of the chair to keep herself upright. Without the support, she feared she would melt into a puddle on the floor.

"Of course it is." The mouth against her chest wandered further down, tickling her ribs. Korra squirmed, but didn't really want to escape. Asami's fingers were still just out of reach... "Well, more complicated versions of the same concepts. It all fits together. Engineering is all about energy. How to move it. How to convert it. How to use it to get a specific job done..."

Finally, at long last, Asami's fingers began to slip forward. The stretch made her freeze at first, but it only took a few moments to adjust to the new, pleasant fullness. "That's all a machine is." Asami began a slow curling motion, catching against Korra's front wall with a well practiced stroke. "A tool you use to channel energy into an action. Our bodies are machines."

Korra's heels kicked at the chair beneath her. Her pelvis tilted forward, trying to take Asami even deeper, rocking so that those talented fingers could keep rubbing against the swollen, heavy spot inside of her. Her clit twitched against the bottom of Asami's hand, and her head swam with brilliant colors. She couldn't form a single word with her slack mouth, but Asami didn't seem to mind.

"Right now, I'm helping you understand dynamics. The study of motion and the forces that cause it. If you convert energy into force, and use the force to apply pressure..." Korra's eyes rolled back into her head as Asami pushed all the way inside. "...you can create motion. And that motion causes friction." Asami's fingers began pumping in and out, thrusting harder, picking up speed. "The more force you apply, the more you accelerate, the more friction you cause. And the more energy you transmit into the next part of the machine."

Korra muscles fluttered, clasping down as a surge of heat rushed from deep within her. The pressure was unbearable, pounding inside of her, throbbing in time with Asami's movements. "I... I'm so close..." she said in a tight, desperate whimper, bordering on the edge of a scream. The fingers inside her felt so good. Left her so full. And the heel of Asami's hand kept rubbing against her clit, scraping over the exposed tip with every push. "Asami, please..."

Asami's warm mouth started kissing back up along her neck, stopping beneath her jaw. Korra trembled as the soft, ticklish strands of Asami's hair danced against her cheek. "But we're not finished with our lesson. We haven't even covered the basics."

"Asami, please." She knew she should plead more fervently, but the words wouldn't come. It was all she could do to form her lover's name.

Soft laughter vibrated against her ear. "If you insist. But I'll be testing you on this later." Asami kissed her one last time and drove her fingers home.

Korra couldn't stifle her shout any longer. It ripped from her chest, burning her throat on the way up. Her muscles clenched and shivered, forcing a flood of wetness out around Asami's knuckles. The bud of her clit pulsed, and the rest of her body locked up tight as all the tension inside of her finally released in blissful waves. She came until she couldn't anymore, levitating off the seat of the chair, arching into Asami and clinging to her for dear life.

"And that's mechanical engineering," Asami sighed into her hair, breathing shallowly as the ripples faded. "Fitting parts and pieces together to move energy where you want it to go. It's simple, once you get the hang of it." She glanced down at her hand, and Korra blushed when she saw how wet Asami's hand was. The fingers nestled inside of her were covered in slick, shimmering glaze. "And you seem to understand fluid flow already. I don't need to define that for you, do I?"

Korra let her head fall forward onto Asami's shoulder. "No. But I'd like you to explain why you're still wearing clothes while my leggings are around my knees and my shirt is half way off. It's not fair."

"Korra," Asami said, clicking her tongue in mock disapproval. She gave her fingers one last shove, and Korra shuddered at the surprise movement, fluttering with aftershocks. "You aren't trying to seduce your teacher, are you?"

"I think it's way too late to worry about that. But you're avoiding my question," she said, repeating Asami's words from earlier. "Why do you still have your clothes on?"

With a sigh, Asami withdrew her hand and left the chair, pushing up into a standing position. She slipped her wet fingers into her mouth as the other hand started to unfasten the belt that highlighted her waist. "Well, when you put it like that... maybe we should study the amount of friction caused by skin to skin contact. I think I've got one more lesson in me tonight."