A/N: I got sidetracked from my running chapterfic by the lovely bennyslegs on tumblr, who suggested: "sherlock finding one of john's hairs on his robes and using it to make polyjuice and then wanking, in john's body, in his bed until it wears off..." Naturally this sufficiently distracted my brain.

This has nothing to do with the Potterlock universe around my fic Expecto Patronum, except that a lot of my background canon is the same. I decided to Sort Sherlock into Gryffindor not only because this worked better if they shared a dormitory, but also because I think the Hat would find the part of Sherlock that wanted to be a pirate and try to cultivate it by putting him with other romantics.

The idea occurred to him at the most inopportune of moments.

Perhaps there had been a flicker of something, earlier in the afternoon, when Professor Slughorn had been in raptures over the many uses Polyjuice potion had served when Harry Potter overthrew Voldemort, but the actual plan, fully-formed and absolutely beautiful, didn't pop into his head until right in the middle of dinner, a spoonful of Yorkshire pudding halfway to his mouth.

He had remarked to John at the time, idly, not quite realising the full implications of the comment, that Slughorn was probably missing quite a few potential uses of Polyjuice potion when limiting his lectures to disguise and avoiding detection.

It hadn't been until dinner that he'd realised what one of those potential uses might be. John had been facing away from him, laughing with James Potter about something to do with the recent Quidditch match between Hufflepuff and Slytherin that had meant something important for Gryffindor's chances in the Cup, and Sherlock – as had become customary when John was not looking at him – was staring at the back of his head and trying not to let Potter see quite how badly he was in love with the Keeper.

Loving John had snuck up on Sherlock quite suddenly. The Sorting Hat's decision to place him in Gryffindor five years ago had been as much a surprise to him as it had to everyone else; the Holmeses were an ancient pureblood line, tied in closely with the Noble and Most Ancient House of Black, and though there had been a few 'aberrations' – his mother's words – in the family the grand majority of them had been in Slytherin. Mycroft had been placed there without a second thought, and while Sherlock had worried every now and again about the role of many Slytherins in the famed Battle of Hogwarts, he had come out of it perfectly intact. Sherlock had always assumed that he would follow the same path, and he'd never imagined anything else.

Professor McMillian had had to help him off the tiny stool, he had been so surprised, and when the shock had worn off, the fear had set in. What would his parents say? What would Mycroft say? By the time he had sat down at the Gryffindor table he'd been shaking in trepidation. How long would it take for them to find out, if he did not tell them?

James Potter had patted him reassuringly on the back, but he had barely noticed anything until a small blond boy he had seen on the train plonked onto the bench beside him, put a hand on his shoulder, and asked him if he was all right. John had comforted him then, cheered him up right through his worry until his mother had written to him rather stiffly to say that they did not mind what house he had been sorted into. By the time the crushing fear of disappointing them had faded, he and John had become friends. By the time they had sat their Ordinary Wizarding Levels together at the end of the previous year, Sherlock had fallen completely head over heels in love with him.

He couldn't say anything, of course. John quite happily told Sherlock the sordid details of his every crush, all of them female and none of them anything remotely like him. He thought he did a relatively good job of hiding it, though he had noticed Potter – who had taken on a role as some kind of mentor to John after his acceptance into the Gryffindor Quidditch team – shooting him glances lately that ranged from speculative to wryly amused.

John was trying to simultaneously eat his pudding and discuss tactics with Potter, but it wasn't until he ran a hand through his sandy blond hair, tugging a few strands of it loose and shaking them away, that the idea hit him with all the force of a rampaging Erumpent.

He could use Polyjuice to turn into John. He could discover the every intimate detail of his best friend's body without ever letting him know, without any risk to their friendship. He found the Keeper's short blond hairs everywhere, and the potion only required one. Godric, he could keep an entire stash of the stuff at the bottom of his trunk and assuage the need he felt when he looked at the blond without him being any the wiser.

Sherlock had never particularly liked Horace Slughorn. At that moment, though, he could have kissed him.

Sneaking into his office and making off with a few bottles of Polyjuice was child's play – he had been helping John break into the broom shed by the Quidditch pitch for years, and they had broken into Filch's office more than once to destroy reports about certain wrong-doings before they were filed – and plucking one of his friend's hairs from his robes was the work of less than a moment. Waiting for Saturday morning when he could have the dormitory to himself, however, was more difficult. He'd never been a particularly patient person, and by Friday night he was so jittery with anticipation that he could not sleep at all.

Not sleeping was a problem that Sherlock had fairly often, though, and so John did not comment when he woke up and found Sherlock still pacing the dormitory in his pyjamas and blue silk dressing-gown but merely patted him on the back in a conciliatory manner on his way to the bathroom. By the time he emerged in his Quidditch reds, their other dormmates having left for breakfast, Sherlock had climbed back into bed and pulled the hangings shut around him.

He cast a Silencing charm even though he did not expect to be disturbed; no-one really used the dormitories for anything other than sleeping and while John would probably come back to check on him after Quidditch practice, that wouldn't be for hours yet and he would have plenty of time to turn back into himself and clean everything up.

Sherlock had spent a fair bit of time in the Restricted Section since having the idea, and he'd read stories about how foul drinking Polyjuice potion was. He'd been ready to pinch his nose and force himself to swallow a terrifically nasty-looking and –smelling concoction, but once he had dropped one of John's hairs into it, the little bottle turned the sandy yellow-blond colour of the hair and gave off a smell like peanut-butter and petrichor. It was very John, somehow, and Sherlock drank it without a second thought.

And promptly dropped the bottle, where it shattered on the edge of his trunk. It tasted fine, but the instant it hit the back of his throat it stuck there, coating his insides, as though it were trying to bleed through the fabric of his oesophagus and into his bloodstream. His stomach roiled. He groaned, curling up into a foetal position as he began to turn inside out; he could feel his hair receding into his scalp and his bones grated against each other as they shifted, setting his teeth on edge. His skin bubbled and stretched and shrank and his teeth moved around painfully in his jaw.

Just as Sherlock began to think he couldn't stand it, it faded abruptly, leaving him trembling on his bed, his hands clutching his stomach – a stomach that felt different somehow. His arms were bigger. He had muscles, he could feel them flexing and relaxing with each compulsive shiver.

His nausea forgotten, Sherlock jumped out of bed and rushed to the mirror at the end of the dormitory, wincing when his pyjamas pulled tight across his shoulders and his newly-defined pectorals.

John's face blinked back at him, only it wasn't John's face. It resembled most closely the expression that the Keeper assumed all those times when he tried to imitate Sherlock, something odd in his posture and the way he was staring into the mirror. Sherlock relaxed his back slightly, tugging at his shirt awkwardly and trying to imitate John's friendly, lopsided grin. His stomach flopped pleasantly, just as it always did when John smiled at him, and something triumphant tugged in his chest at the knowledge that he could have John smile at him like that anytime he wanted now – or not smile at him. He could do more with John's face, too… Sherlock tipped his head to one side and widened his smile slightly, raising one hand to lean closer to the mirror, eyes fluttering as though in anticipation of a kiss –

Letting his lips touch the cold mirror would break the fantasy, though, so at the last minute Sherlock pulled away, shrugging his dressing gown off his shoulders – with a bit more difficulty than usual, as John's shoulders were beautifully broader than his own – and lifted an eyebrow. He didn't try to be seductive or flirtatious, because John would never do that. He wanted John as John, wanted a John so comfortable with him that he knew Sherlock would react to anything he did. Because he would – he had been half-hard since the nausea had faded and he had realised he was feeling John's muscles tense and his hands clench and his teeth inside his mouth, and lifting his pyjama t-shirt away from John's Quidditch-honed torso caused more than a flutter in his own abdomen.

His skin tingled; he had seen John without a shirt before but this was different. He knew he could have this – he could touch, he could splay his fingers over John's stomach and feel the muscles tense and ripple, draw his hands up until his fingers – shorter than usual, tanned, with callouses over the palms from holding a broomstick – brushed against the swell of his pectorals and danced over his nipples. John's nipples weren't as sensitive as Sherlock's, but simply the touching of them still sent sparks spinning through his body to his groin.

Sherlock staggered back to the bed, making sure to pick up his shirt and dressing gown and yanking the hangings shut behind him. He smelled like John, too; arousal had made him begin to sweat and he smelled just like John after Quidditch practice on a Wednesday evening, when he would collapse into Sherlock's lap in the common room and snatch his book from his hands. Sherlock would pretend to be annoyed, but he knew he didn't do it very well. He lifted his arms, revelling in the stretch of muscles built up after years of gripping a broomstick and catching a Quaffle, and got his nose as close to his armpits as he could and breathed in the smell of John.

He couldn't keep his hands away from his – John's – skin, the feel of it on his fingers, the way they were John's fingers too. He wanted more, and yet it was far too much. He sighed, and it was John's sigh, and the surprise of it startled a moan from his lips and it was John's moan. He gasped. "Sherlock," he murmured, the noise echoing slightly within the confines of his Silencing Charm. But it was John's voice and suddenly Sherlock was so hard it felt as though the seams of his pyjama bottoms would split, made as they were for Sherlock's slim hips. He lifted his hips, feeling the strain in his newly-muscled thighs, and worked the pants off until John's cock sprang back up and hit his muscled belly.

Sherlock moaned freely, rejoicing in the sound of John's voice. A tiny string of pre-come stayed behind on the golden lines of John's abdominals, linking it to the perfect head of his perfect cock. He propped himself up on his elbows just to look at it for a while. It was nestled in a bed of lush sandy curls that Sherlock wanted to wind his fingers through, rising hopefully towards his stomach. It was shorter than Sherlock's, but thicker, and it throbbed like nothing he'd ever felt before. His own didn't do that. He wanted to touch it, but he didn't want this to be over – there was an entire body to learn, to experience, he didn't want to rush it.

But he would be like this for an hour.

Reverently, he slid John's hands down his stomach, lost in the paradox of it – they were John's hands on his stomach as though they had never felt anything so beautiful, or were they his hands on John's stomach, feeling the bumps and rises of his incredible body?

"Sherlock," he moaned again, his hands nestling into his pubic hair, feeling the heat from his cock as it twitched at the sound of his name so passionately in John's voice. Suddenly frantic, he fixed his hand around it and pulled, groaning desperately at the feeling of the broom callouses on his palm as he stroked it up and down his – John's – cock. He tried to go slowly, dipping his other hand down to fondle John's testicles, larger than his own and infinitely more sensitive, but it was useless; his hand moved completely outside of his control, reinforcing the fact that it wasn't his hand, it was John's, and that just ramped his pleasure up past the level where he could restrain it; pre-come was dripping steadily from the head of his gorgeous prick and onto his navel and the sensation of it sliding across his head was so unbelievable that he bit his lip – John's lip, thinner than his own, tasting of John's sweat and not Sherlock's – and threw his head back and groaned one last time in John's voice, pleasure shaking along all of John's muscles and turning his vision white.

It hit him right in the middle that he had brought this pleasure to John's body, an idea that made him shudder with another wave of it. John's body was lying on his bed, wracked with the aftershocks of orgasm with come pooling on his stomach, because he had made it so.

He sighed contentedly and stretched out his sated muscles, wincing as a trickle of come ran down his side at the movement and wiping it with his fingers before it reached the bed. He would clean up afterwards, of course, but he didn't have the energy quite yet and he didn't want to be lying in a pool of his own spunk until he did –

But it wasn't his own spunk, Sherlock realised, spreading it with wide eyes between his fingers. It was John's. He had John's come all over his stomach, as though the Keeper had leant over him and finished – because of him, he reminded himself – onto his skin.

Helplessly, unable to resist, Sherlock put his fingers to his lips and licked the salty fluid away from them. He hadn't expected it to taste any different from his own, but surprisingly it did, slightly sweet and other. It really wasn't at all awful. He trailed his fingers through it again and brought them back to his lips, painting them with it. He wondered what John's lips looked like covered in his own come, but he certainly couldn't venture out from his cocoon of spelled safety to the mirror like this in case one of their other dormmates walked in. He couldn't have them approaching John to talk about his weird masturbatory habits.

He reached for his wand on the bedside table, licking his fingers clean so that he didn't get John's come on it. He'd tried this spell before, but never on anything big.

"Speculo!" he muttered softly.

The canopy above his head shimmered and changed until it reflected him back to himself, John's body stretched out indolently on red and gold Gryffindor sheets, come splattered over his abdomen and smeared over his lips.

Sherlock whimpered. John stared up at him from his bed, wanton, beautiful. As he watched, his cock twitched and stirred again and Merlin, it was even better this way, when he could see John's body reacting to his touch, see John's hand sliding down his stomach through the come already pooled there and past his penis as it hardened before his eyes, trailing along his testicles and back. It felt forbidden and just a little voyeuristic, but Sherlock had promised himself he wouldn't think about the questionable ethics of what he was doing until it had finished, so he pushed that thought out of his head and skated his fingers over his perineum instead, forcing a grunt from his throat. He wanted to know every inch of John's body better than the Keeper knew it himself. There was a tiny thought in the back of his head that one day he might get to touch it as himself, without the use of magic, and he wanted to know exactly where and how to touch to get the best reaction.

His fingers, thicker than he was used to, grazed over the tight furl of John's hole, drawing a startled gasp from his throat, one that sounded exactly like the gasp John made whenever Sherlock stole food out of his hands on its way to his mouth. He couldn't resist pushing in, just a little, John's come slicking the way of his fingers, the stretch echoing pleasurably through his groin. His other hand pushed against his penis, his eyes fixed on John's in the mirror. The Keeper's blue eyes bored into his as he worked his finger deeper, deeper, but John's fingers weren't long enough and he couldn't reach his prostate – not that it mattered, because the sight of his best friend reaching his own hand into his arse as though preparing himself for Sherlock was irresistible, and his breath was coming out in desperate little gasps and he was so hard again, was it possible that John's libido was even more rampant than his own, even when the tiniest little flirtatious gesture from the Keeper was enough to wake his cock?

Sherlock groaned again, throwing John's head back into his pillow; the bed smelled of Sherlock and he smelled of John and his nose was full of the smell of the two of them together and John's cock was hard in his hand that was John's hand, and he was touching John and John was touching him, his fingers in John's arse and John's arse around his fingers, and oh Merlin oh fuck it felt incredible, and he wondered if John had ever tried this himself because his body was so responsive and the idea of it, of John actually lying in his own bed with his fingers – three now – stretching and curling inside himself and maybe imagining that it wasn't his fingers at all but Sherlock's fingers, Sherlock's cock, and oh, Godric!

He arched his back as he came again, feeling it shoot right up his chest, a strangled noise ripping its way out of his throat, and he wondered wildly whether his own muscles would ache when he came back to his own body and left this one behind, feeling empty already at the thought of leaving it.

The pleasure of his second orgasm in John's body left him breathless and exhausted, panting and trembling on his bed. As the climax faded, the fact that he hadn't slept the previous night came back to him in full force. He dragged his fingers lazily through the fresh come over his stomach and chest, sighing happily, and before he could stop to think whether it was a good idea, he had dozed off.

When he woke it was to his own erection bending eagerly towards John's congealed come on his once more pale and flat stomach. He hummed lazily, reaching for his wand to renew the Silencing charm and get rid of the mirrored ceiling.

Curiosity got the better of him and he ran his fingers through the somewhat disgusting mess on his chest and tentatively touched it to his lips. It still tasted like John. His eyes drifted down to his erection. He couldn't help but wonder what they would taste like together, if he could just…

He reached down and gripped his cock, his own long fingers so familiar on his length, remembering the feeling of John's cock in his hand, John's calloused hands on his cock. He gasped and wriggled, wincing at the slightly unpleasant sensation of cold semen sliding across his abdomen, not quite willing to dispense of the evidence that something of John had been in his bed –

The door banged open. Sherlock froze, his hand gripping hard at the base of his cock. He was cocooned in his Silencing charm, but he didn't want to move in case whoever it was saw his bed hangings shifting and guessed that he wasn't asleep. He held his breath, straining to hear who it was that had interrupted him and was now toeing off his shoes.

"Sherlock?"

He cursed under his breath at the sound of his best friend's voice, but kept the Silencing charm up in the hope that John would assume he was still sleeping. His cock trembled in his hand as John continued, "Lunch is up, if you're awake. I brought you some tea."

John's footsteps came damnably closer to the bed; Sherlock panicked, scrambling for his wand to release the Silencing charm, yanking the sheets out from underneath his body to try and cover himself, but it was too late.

"Sherlock, are you – oh!"

The curtains flew open; Sherlock yelped, sitting up quickly, his wand flying across the room as he struggled to cover his erection. John let out a surprised shout, his mouth falling open for a moment before he seemed to recover himself and turned around abruptly. "Merlin, Sherlock, I'm so sorry! I thought you were sleeping!"

Sherlock finally managed to get the bedsheets up to his chest, smearing the almost-dry come under his hands with a wince. "Never mind, John," he tried to say, his voice coming out embarrassingly high-pitched. "Don't worry about it." He cleared his throat desperately.

John didn't move. "No, I… I really am sorry," he repeated, fidgeting with the teacup in his hand before placing it firmly on Sherlock's bedside table. "I brought tea, I thought – but obviously you're not – I'll just go, shall I?"

Perhaps strangely, John's bumbling awkwardness almost made Sherlock want to laugh. "Probably for the best, John," he said, wishing that he would not, wishing that John would turn back and offer to help him out in that flippantly awkward way that he had.

"Right," John agreed instead, nodding briskly. "Yeah, leaving. I'm just – ow! Is that – Sherlock, there's glass on the floor!"

Sherlock peered at John's feet to see the shattered remains of the Polyjuice potion's vial. "Oh," he muttered awkwardly. "I… got a Sleeping Draught from Madam Pomfrey, I must have knocked the bottle," he lied quickly. "I'm sorry."

John bent and picked up the largest piece, potion still sticking gloopily to its curve. "Obviously it didn't work, because that didn't look like sleeping," he remarked wryly. Then he frowned. "That doesn't look like a Sleeping Draught. That looks like…" he brought it to his nose and sniffed it. "Peanut butter," he assessed, turning back to look at Sherlock, who hugged his knees closer to his chest reflexively. "Sherlock, is this Polyjuice Potion?"

He tried to look innocent, but apparently he wasn't quick enough; John assumed a look of outrage. "Sherlock! That's so illegal, you didn't even – you weren't wanking in someone else's body, were you? That's a huge invasion of privacy! Godric, that's wrong on so many levels!"

Sherlock bowed his head ashamedly, hoping that John would leave it at that, but apparently he had exhausted his supply of luck for the afternoon, because John smelt the remnants of the potion once more and his frown intensified. "Hang on," he said slowly. "Peanut butter – doesn't the Polyjuice potion smell like something that reminds the drinker of the person they're – whose hair did you have access to who eats peanut butter every morning, except for…"

Once again, he wasn't quick enough to hide the flush and the guilty expression, and John sat down on his own bed, gaping at Sherlock, turning inexplicably red. "You used my body? You were wanking in my body? Sherlock, what kind of sick experiment –"

"It wasn't an experiment," Sherlock said quietly. "I'm sorry, John. I just thought if I… you weren't supposed to find out. I thought this might make it easier –"

John's face had surpassed red and gone for purple. "Easier? What, you thought it would be easier to wank if you did it with my body?" he shouted.

"No," Sherlock protested wildly. "I thought it might make it easier to be around you all the time and not be able to have you, to listen to you fantasise about people who are never me, to be your friend and just be happy with it and not always wish for more."

There was a shocked silence in which Sherlock debated the relative embarrassment of crossing the dormitory completely starkers in order to find his wand and hex himself into oblivion. He hadn't meant to say anything like that. Now John knew. Now, surely, everything would be ruined – John was too nice to not be friends with him, but things would be different, this would always be hanging between them. He closed his eyes and buried his face in the sheets covering his knees.

"Sherlock," John said, so quietly he almost didn't catch it. "Are you saying you… you actually like me? Want me?"

He lifted his head in shaky incredulity. "Like – John, of course I like you. Of course I want you. Have you ever spent ten minutes in your own company? It's impossible to talk to you and not fall utterly and completely in love with – mmmpfh!"

Sherlock gasped as John's mouth was suddenly covering his, knocking him back onto the bed; a now-familiar stocky body was climbing over his, calloused palms cupping his jaw through a clumsy but definitely enthusiastic kiss. Not daring to break it to ask what was happening, Sherlock grabbed John's back and held on.

Eventually the Keeper pulled away, resting their foreheads together and panting. "You mad bastard," John told him, his tea-scented breath blowing across Sherlock's face. "I've been in love with you for years."

"But…" Sherlock propped himself up onto his elbows as John shifted so that he was straddling him, sitting calmly in his lap. "But you were talking just the other day about how pretty Frankie Spinnet is! I thought you…"

John shook his head frantically. "I only ever said things like that to see if I could make you jealous," he said, his hand still cupping Sherlock's face. "You never reacted, but I could never stop imagining that maybe one day you'd say something, or blush, or get annoyed –"

Sherlock growled, surging forwards and pressing their lips together again, John's Quidditch robes warm under his hands, his mouth hot and welcoming. "I was always annoyed," he admitted fiercely, clutching at John's hair and pulling his body in towards his own. "None of them deserve you, none of them could ever feel anything like what I feel for you."

John laughed delightedly. "All those years of thinking you were just… asexual, or something," he mused, bringing his fingers up to draw idle patterns on his chest. "Merlin, I love you, you nutter."

"John," Sherlock answered immediately, catching the Keeper's hand in his own and holding it to his heart. "I love you too."

John's hand slipped from his own, sliding down his stomach until it reached the point where the sheets covered him. "Right," he said, his voice bright and teasing, "why don't you show me exactly what you were doing to my body, then?"

Sherlock couldn't help but smile. "Didn't I always say you were smarter than the rest of them?" he remarked, leaning sideways and pulling the hangers shut once more.

John laughed again, and Sherlock kissed him happily and prepared to break apart the tanned and muscled body in front of him into bliss for the third time that morning – and, he thought gleefully, definitely not the last.