A/N: Would y'all like some porn? Of course you would. Enjoy!

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baby don't stop

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It's been a fact of their lives for as long as either of them can remember: what Itachi can't figure out on his own, he learns from Shisui.

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Itachi is a genius; nobody's questioned that since before he could walk (at the age of six months, if anybody's keeping count). There's never been much that causes him difficulty, nothing that takes his big brain more than a few minutes or a few seconds to grasp and replicate. It'd probably come as a shock to certain people that Itachi requires instruction in anything at all.

But Shisui knows damn well that nobody can be a genius in every area, at least not right out of the gate. So that's where he comes in.

"Shi—"

Shisui nips at his ear, a warning to stay quiet, and Itachi shuts his mouth—it takes a visible effort but he does it anyway, and Shisui feels an absurd twitch of pride as he returns his attentions to the skin of Itachi's neck. (They're learning the fine art of not leaving marks today; expert demonstration followed by practice is the order he's come up with for these things.) Without removing his mouth from the pulse point just below Itachi's ear, Shisui nudges Itachi's legs apart and slots their hips together. (He's a big believer in positive reinforcement making lessons stick better.)

Itachi's mouth opens again at the movement, instinctive, but he bites his lip before any sound escapes. (See? Shisui pats himself on the back. Positive reinforcement.) His eyes flutter closed as Shisui moves lower, to the juncture where neck meets shoulder. They've still got all of their clothes on, even as the heat in the room rises, since Shisui is also a big believer in taking things one step at a time.

He doesn't think Itachi agrees with that approach, given the way his hips are twitching like he wants really badly to arch up against Shisui's body, but hey—he's the one who asked for Shisui's help in the first place. He's not really in a position to complain about how he gets it.

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He's not sure how long it's been since Itachi asked him for lessons on sex, which is weird because at the time Shisui was pretty sure his head temporarily exploded.

(Not that Itachi had been that blunt about it, of course. There'd been the usual amount of hemming and hawing the boy genius employed whenever he was forced to admit that he wasn't totally and immediately proficient at something, couched in phrases like "practical demonstration" and "mutually beneficial", which went on for about ten minutes before Shisui grasped what he was going on about and his head, as aforementioned, exploded.)

After he'd gotten over the initial shock Shisui had been forced to admit that okay, maybe he should have seen this one coming. It figures that Itachi would run smack into the brick wall of puberty earlier than everybody else, because he's done everything earlier than everybody else. And if Shisui was being honest with himself, yeah, he'd noticed Itachi staring at him more than usual lately (because Itachi is always staring at something or another, it's just what he does), and with more, uh…intensity. Shisui's failure to recognize that look as reminiscent of his own at that age (specifically, upon discovering the Icha Icha author had written an earlier series of books involving only men) was probably further proof that he was no longer The Vaunted Uchiha Prodigy and hadn't been since approximately five minutes after Itachi's birth.

The only way to atone for his obvious neglect, Shisui had decided, was to devote himself to teaching Itachi everything he knew.

(He thinks he's turned out to be a pretty good instructor in that regard, and if the noises Itachi's learned how to smother are any indication, his current student wholeheartedly agrees with him.)

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Of course he's probably still going to hell for this, he reflects just before he swallows Itachi's cock.

A soft sound escapes Itachi's mouth before he strangles it, but Shisui lets it go; he figures he can make some allowances since this is their first time doing this. (And besides, he's learned that the key to keeping someone engaged with learning is to make sure they enjoy the process of it.)

He doesn't give Itachi much time to adjust to the new sensation before he inhales through his nose and takes him in deeper, running his tongue along the side of Itachi's cock as he sucks. Out of the corner of his eye Shisui can see Itachi's hands fisting in the sheets, his knuckles going white with strain. He almost feels bad—if it were anybody else he would be going slower with this, giving them time to recover a bit before he ramped up the pleasure, but Itachi can handle it. He's a prodigy, after all.

With that in mind Shisui steels his gag reflex and sucks hard, moving forward until Itachi's cock is nudging at the back of his throat.

Itachi lets go of the sheets so fast Shisui almost doesn't see him move; his hands fly to his mouth, clamping down to keep any sound from escaping. Shisui's pretty damn impressed, so impressed that he promptly decides he's going to give Itachi the best orgasm of his life. (So far, he amends silently. Always leave room for improvement.) He starts moving his head up and down in long, rhythmic strokes, swirling his tongue around the head before diving back down, over and over until Itachi's face is so red with the exertion of keeping silent that Shisui absentmindedly worries he might pass out.

As it happens, he comes down Shisui's throat first.

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It's a thought Shisui revisits a few more times over the course of their "lessons"—the going-to-hell question. After all, Itachi's not even old enough to drink yet (and isn't that some bullshit, that he's old enough to kill people for a living but apparently not old enough to be trusted with alcohol).

There's no such thing as "too young" for a prodigy.

Shisui winces; even in his head the words sound twisted.

He's willing to admit that the whole thing is probably fucked up if he thinks about it too hard. Not the cousin factor; for one he's still not totally sure how they're related, the compound being a haze of "cousins" and "aunts" and "uncles" because the Uchiha family tree is a fucking knot, one not even the Sage himself could untangle. Nobody really keeps track of that shit anymore and nobody particularly cares; clan politics prioritize the purity of the bloodline above all else, so as long as you're not fucking your sister almost anything goes. (And Shisui knows he's got that base covered.)

So, no, the potential incest isn't really losing him sleep. Shisui's marginally more concerned about the age issue, but even thinking about that kinda pisses him off, because it comes back to the same damn double standard: Itachi's old enough to kill people for a living. Why shouldn't he be old enough to fuck around with someone, if that's what he wants?

(Especially if that person is Shisui. Hell, maybe only if it's Shisui, because he's always looked out for Itachi and he always will. He can be trusted with his cousin's delicate, probably messed up teenage genius psyche in a way some rando Chunin could never be.)

Ultimately Shisui decides it's a pretty pointless train of thought. If only because in the weighing of his sins, being a career assassin is probably going to rank higher than stealing his younger cousin's virtue or whatever. That just seems like common sense.

Besides, he's never been able to say no to Itachi in his life. So really, what else is he supposed to do?

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The fun part about living in an insular clan compound made up exclusively of nosy relatives is that when you do want to fuck around with your cousin (for the sake of his continuing education, obviously), it's borderline impossible to manufacture a situation in which you can both be alone and undisturbed for the amount of time such an initiation deserves.

Did Shisui say "fun"? He meant "eye-gougingly heinous".

As usual, the Nakano comes to their rescue. Or more specifically, a shallow cave set right near the river's mouth.

Shisui does have a moment of mind-numbing panic when he brings Itachi to the place and realizes, somehow for the first time, that he's about to deflower his cousin in a cave; then another, worse moment where he realizes Itachi is realizing that Shisui's about to deflower him in a cave. But Itachi, being Itachi, doesn't react beyond a single eloquent raise of the eyebrow, and Shisui, being not an idiot, at least thought ahead enough to put down blankets and such. If you tilted your head and squinted (heavily), you could almost call his efforts romantic.

It's not a massive space, but there's enough height that they can both sit upright inside the cave without smacking their heads on stone. They kind of sit there awkwardly for a minute, Shisui struggling to recall his lesson plan for this situation and mostly only succeeding in quietly hyperventilating.

Itachi looks like he's about to say something well meaning but accidentally soul-crushing, as he is wont to do, when something occurs to Shisui and suddenly the day is saved. He grins, making Itachi tilt his head in a silent question.

"You know what's great about caves?" Shisui says. Itachi blinks in a manner that somehow conveys suspicion.

"What is that?"

Shisui leans over to kiss him so quickly that Itachi knocks his head into the cave wall, but he doesn't resist it, his mouth opening almost immediately under Shisui's.

Then, just as suddenly, Shisui pulls away until their noses are barely touching.

"There's nobody to be quiet for out here," he murmurs, and is satisfied when Itachi visibly shivers.

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Instruction isn't a one-way street. Shisui's learned plenty during their recent time together.

Such as: Itachi really, really likes being praised.

(Mark Shisui down as not especially surprised but gratified all the same.)

Shisui's got him on his knees and elbows, sweaty hair hanging around his face where it's escaped from his earlier ponytail. He kind of wishes he could see Itachi's face better, but all in all this is a good angle. Especially when Shisui's currently three fingers deep inside of him and bent over the curve of his spine, murmuring in Itachi's ear as his fingers do their work.

"You're doing so good," he breathes, and it's not even lip service, because Itachi would kill him if it was. "You're opening right up for me."

Itachi shudders, the movement driving him further back into the press of Shisui's hand, and his exhale comes out ragged. He's practically dripping he wants it so bad; Shisui remembers being his age and empathizes. He's feeling it too, after all.

But he's patient. He waits. (You can't expect people to learn if you just hand them everything.) Finally he can just see Itachi's tongue dart out to wet his chapped lips.

"You can—" he manages, but Shisui interrupts.

"I know I can, you've been ready for ages. But what do you want?"

Itachi twists around to look Shisui in the eye, his expression hazy and open and utterly certain.

"You," he says simply.

Shisui's throat makes a clicking noise when he swallows. He wants to say you got me or something else equally cheesy and ridiculous, but that would take this in a whole different direction and neither of them are here for that right now. (Another good lesson to learn: the importance of timing.)

So instead Shisui takes a deep breath, sends up a prayer that he doesn't come in under ten seconds because that suddenly seems like a very real possibility, lines himself up and fucks in and listens to his cousin moan.

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More observations, to be recollected and analyzed when Shisui isn't high as hell off endorphins:

First, Itachi follows instructions well. Which isn't exactly new information, but Shisui's particularly impressed that he manages to keep the rules straight in his head when all other coherent thought is being demonstrably fucked out of him.

Second, given the freedom to be so, Itachi is actually pretty loud about sex. Especially when he's been on the edge for god knows how long but Shisui hasn't told him he's allowed to tip over that edge yet, so all he can do is move and moan and take it until Shisui finally has pity (which he knows isn't really pity so much as the fact that he's about to come so hard he sees stars and doesn't want to be the one to finish first when his partner is a literal virgin) and says—

Yes, now, I want to feel you—

—at which point Itachi comes hard enough that Shisui's pretty sure that blanket is ruined. At the very least it's going to need to be their designated Sex Blanket. (Because one should always, always have a designated Sex Blanket.)

They're lying on that blanket now, still breathing more heavily than shinobi stamina would seem to suggest. Shisui, for his part, is kind of stunned. How is this fair? How can it be this good when Itachi doesn't even know anything yet? How terrifying is he going to be once he's learned everything Shisui has to teach him?

Shisui's not sure how long he ambles down that (slightly terrifying) path of thought, but suddenly there's a flash of dark hair and then Itachi is on top of him, looking down with those calm black eyes.

"Yes?" Shisui manages after a moment.

Itachi raises an eyebrow again, which is apparently his preferred method of communication when he's unwilling to stoop to actual speech. Shisui gets the hint nonetheless, because Itachi has shifted against him and good god, the stamina of this idiot. Shisui's created a monster. Shisui is going to break. He's opening his mouth to say something to that effect when Itachi leans down, his lips brushing Shisui's ear in a way that feels uncannily familiar.

"Well?" he murmurs. "'Demonstration followed by practice', is that not your method?"

It takes Shisui's brain a minute to catch up (to be fair, those endorphins are a bitch when it comes to rational thought), and when it does it promptly explodes once again at the world of possibilities that have just opened up before him.

"Fuck me," he says. "You really are a genius."

"So I've been told," Itachi replies, and proceeds to follow Shisui's last instruction to the letter.