"If you know the distance between the telegraph poles and count how many we pass in a minute, you can tell the speed we're travelling at," Sherlock explained laboriously. John frowned.
"Or you could just measure the speed with a speedometer. But I think I get it," he said after a pause. The Hogwarts express rattled through the darkening countryside; the dark silhouettes of rolling hills were just visible in the dusk and the pale sun was halfway below the horizon.
"It's boringly simple," Sherlock groaned, stretching out his long legs across the compartment. They were the only ones there. "We're over halfway there, if the landscape is telling us the truth. I'm going to change into my robes." When John moved to leave, he caught him from the shoulder. "You don't have to go," he said, curious. "We changed in the same room all summer."
"I'm just stretching my legs," John replied hurriedly. "I'll change later."
When John had left, closing the door behind him, Sherlock flicked his wand to close the blinds. As he turned to reach for the overnight bag where his uniform was stored, he stroked all thirteen inches of it. It was a rare one, Ollivander had said - not only because it was cedar with a coral core, but also because it was not made by him. Sherlock smiled to himself, thinking how similar the wand was to him, from the rarity to the fairly ordinary wood and startlingly peculiar and powerful interior. He pulled down his back and began to change.
When John reappeared (with his arms full of cauldron cakes and a bottle of pumpkin juice under each arm), Sherlock was in his school robes. The buttons on his shirt strained against his torso and John tried not to look like he was pointedly staring at his friend's tight shirt.
"I got food," he managed, averting his eyes. Sherlock hadn't noticed; he was too busy levitating his bag. With a sharp twist of his wrist he'd replaced it with a puff of smoke, which promptly vanished. "Sherlock?"
"Oh, I beg your pardon. Food? Excellent." He put his feet up on the seat and took a cauldron cake. John dropped the rest onto the seat opposite and sat down next to the food.
"I bumped into Molly," he said conversationally. Sherlock groaned.
"Hooper? What does she want now?"
"You, apparently." John smirked and uncorked a bottle of juiced and began to drink. Sherlock watched as the amount of liquid in the glass container went down. "I said you were in the front carriage."
"I'm not in the front carriage."
"It was meant to deter her," John explained. "She seems keen. Why won't you take her?"
"Not my area of interest," Sherlock sighed. "You call yourself my friend and don't know something like that?"
"I guess I do subconsciously . . . but Sherlock, surely you get, um, cravings for stuff? And you can quench that craving with anything, anyone -"
"Don't be absurd. Molly Hooper? She's an overgrown nine year old."
"She's pretty good. Good looking, that is," John said quickly. Sherlock snorted.
"If you like porcelain dolls with snub noses."
"You're being horrible now, Sherlock. Don't do it, you're nicer when you're being nice."
"I prefer being like this. It's how I be me." Sherlock looked down his nose at John. "I don't want to have any kind of relationship with Molly Hooper. End of."
John sighed. "It was worth a try," he muttered. The compartment door slid open and a tanned figure entered, shoving aside the food and sitting down.
"Alright, Sherlock?"
"Greg," Sherlock said indifferently. "Why are you here?"
"Saying hello," he replied gruffly. Greg Lestrade's quiff of mousy hair was ruffled to perfection and he was already in his gryffindor robes. A gleaming prefect badge shone on the older boy's lapel. "How's things?"
"How are things," Sherlock corrected idly.
"Please," John said, "this isn't the time." He took a bite from a cauldron cake, and Lestrade took a bottle of pumpkin juice.
"Why not?" Sherlock said sharply. "He should know how to speak."
"I can speak, thank you," Lestrade huffed. "What am I doing right now, if not speaking."
"Arguing," Sherlock retaliated. "Don't argue with me, it doesn't work." He looked at his long feet.
Outside, the sun had set. The sky was dark purple-grey. John ate another cauldron cake, not before offering one to Lestrade, who accepted, before leaving to assemble with the other prefects. John checked his watch. "I should probably change," he said, getting up to leave. Sherlock made no reply, so he left silently before finding an empty compartment to change in. It took him awhile, and when he did, it was pitch black outside. When he pressed his rounded nose against the window he could see thousands of pinpricked stars inlaid in the sky. He locked the compartment and shut the blinds, stripping out of his muggle clothes and donning shirt, trousers and a long robe. Admiring his reflection in the train window, he frowned. Something was missing. He pulled off the robe, put on one of his customary jumpers, and put it on again. Much better.
"John? I asked you to pass the pumpkin juice." Upon arriving back at their compartment, he found Sherlock lying exactly where he'd left him.
"I was changing," he said. "I wasn't here."
"You should have been," Sherlock grumbled. "Can I have it?"
"Alright."
"Open it for me, please?" He watched imploringly as John tugged the cork from the bottle with his mouth.
"There." John passed the bottle to him. Sherlock lazily drained it in one go, and the door opened. John opened his mouth to warn Sherlock a second too late.
"I've been looking all over for you," an eager voice said. "Hi."
"Hello, Molly. Cauldron cake? Please don't feel the need to make conversation," Sherlock added as he saw her move to talk. "Actually, forget the cauldron cake. I mean to tempt Mycroft with it this evening. Even as junior under secretary to the minister, he should have some breaks from the diet." He smirked at Molly, scooping up the cakes. "Sorry."
"You're not," she said heatedly, fleeing the compartment.
"That was well-handled," John sniffed. "Whose shoulder will she be able to cry on now you've upset her?"
"Oh, I can think of plenty of people who'd comfort her. Anderson, Gay Jim -"
"I'm quite sure Jim isn't gay, Sherlock. Just because he's your biggest academic rival doesn't mean you can attack him!"
"I'm not attacking him. And anyway, he wears product in his hair. What straight man wears product in his hair?"
"Me," John said firmly, blushing slightly. "Look, leave it."
"Maybe I shall," he said curtly.

When the Hogwarts express pulled up in Hogsmeade station, John and Sherlock disembarked together. The familiar calls of "firs' years over 'here!" echoed through the gas-lighted station and the pair walked up to the horseless carriages, Sherlock dragging them both away from the carriage holding Molly and 'Gay' Jim. Sherlock smugly observed that though Molly was leaning on Jim, his attention was on Lestrade, who had obviously not troubled to buy new uniform, and was hence sporting extremely tight trousers. He has one fine arse, Sherlock found himself thinking as he followed Jim's gaze, then reddened and shook his head.
"You okay?" John asked, stepping into an empty carriage. Sherlock sat opposite him.
"Never been better," he said, straightening the folds of his robe and his long ravenclaw scarf. The carriage five ahead of theirs lurched forwards. "Looks like we shan't have any compa-"
"Do you mind if I join you?" A smooth, sharpened voice cut through the darkness; in the glow of the lamps Sherlock could see an angular face.
"Of course not," John said, moving next to Sherlock to make room for the stranger. When she stepped into the light, John heard Sherlock's breathing change.
The new passenger was a girl - not a little girl, like Molly, but a womanly girl with small breast and curved hips. She'd coloured her lips deep crimson, her eyelids azure, and her uniform showed her to be slytherin. Sherlock drank her in with his eyes - 32", he thought as his eyes skimmed over her chest. 24" waist, a voice in his head whispered. 34" round the hips. He took in her tight pencil skirt. That's a better arse than Lestrade's. He almost hit himself then, but refrained from doing so.
"I know who you two are," she said after a few seconds. "The famous Sherlock Holmes, and John Watson."
"I'm not famous," Sherlock said stiffly.
"Everyone knows your name since you invented that potion last year. What was it again?"
"A potion of concealment," Sherlock said. "It was easy."
"Very smart. You are smart, aren't you?"
"I'm in ravenclaw," he said, as if that settled it.
"But of course." She extended a red-nailed hand. "Irene Adler. An absolute pleasure to meet you." There was silence as she surveyed him. "What year are you? No, wait, I know this. Fifth?"
"Correct. You?" His eyes met hers for a brief moment.
"Seventh. I'm surprised our paths haven't crossed before," she said, raising a heavily pencilled eyebrow. Sherlock smiled cautiously.
"I'm not particularly." The carriage crunched over the gravel of the Hogwarts front drive, drawing to a halt. "Well, good evening . . ." He looked at her bright lips, coughing slightly. ". . . Irene."
"A pleasure to have met you," she grinned lazily, hopping out of the carriage. "Good evening, Sherlock."
John turned to Sherlock when they had entered the great hall. "What was that?"
"What was what?"
"You just ignored me for a whole coach journey. And," he added, "I thought you said you weren't interested in women."
"I'm interested in this one," he smirked, tightening his scarf. "Come on, to the feast."

Throughout the meal, John craned his neck to watch Sherlock's actions. His friend was surrounded by other ravenclaws, but he'd never seen him look so alone. John stirred his mashed potato with his fork. After all the cauldron cakes, his appetite was ruined. Or was it because of the Irene girl?
John shook himself. No. Was he jealous? He tried to mentally laugh it off, but it stuck to the inside of his brain. Jealous, a little voice said. Jealous. "Stop it," he muttered, hitting himself. Greg, seated next to him, tapped his shoulder.
"You okay, Watson?"
"Me? Oh, yeah," he replied quickly. "Look, I'm just going for a walk." John slipped off the bench.
"You haven't even had any beef yet!" Lestrade countered.~
"Not hungry," John muttered, striding down the isle between the gryffindor and ravenclaw tables. As he passed Sherlock, his arm was grabbed.
"Where are you going?" Sherlock demanded imploringly.
"Out," John replied.
"I feel the need for fresh air too," his friend remarked. John sighed.
"I want to be alone."
"Me too. Why don't we be alone together?" A ravenclaw girl sniggered, but shut up at the look John gave her.
"Fine," he said curtly, walking again.
The duo exited the hall, many eyes on the backs of their heads. As they passed through the entrance hall, their footsteps echoed round the tall ceiling. In silence, they ascended to the next floor via the grand, sweeping staircase. When on the next floor, John walked ahead, turning his head occasionally to check Sherlock was still with him, before darting into an empty classroom, pulling Sherlock with him.
"What was that for?" Sherlock asked coolly.
"Okay, Sherlock. I don't know what's going on, but that Irene girl. Why were you so entranced by her?"
"The same reason you're entranced by Professor Gillyflower." John pinkened.
"No."
"What?" Sherlock smirked, not for the first time that evening.
"No. You aren't attracted to her, are you?"
"Not sexually. She just has an aurora of . . . trouble." Sherlock's hand was in his pocket, stroking his wand.
"Why is that interesting? Would you be more interested in me if I had an aurora of trouble? Interested as a friend," John added quickly.
"I think we may have another case on our hands soon, John," Sherlock grinned. "And I'll wager the blood will be on Irene's hands."

Weeks passed in the blink of an eye; John worked hard by day, studied by evening, but his nights were spent roaming the castle with Sherlock exploring new corners and passages. Even though they'd attended the school almost five years, there were still secrets to be discovered - too many secrets for John's liking, but barely enough to quench Sherlock's appetite. They'd persuaded Greg to join them, once, but he generally declined when the pair invited them on a late-night stroll.
"I'm meant to responsible," he'd huff, or more often, "I don't want to be the third wheel on your beautiful relationship."
That night, however, when they met at their usual spot, it wasn't to roam the castle.
"Ever been to the Ravenclaw common room, John?" Sherlock asked, striding up to his companion. John frowned.
"Am I allowed in there?"
"Of course not," Sherlock scoffed. "Come on." He started walking briskly.
"Sherlock," John protested, following him anyway.
"I want you to see something," Sherlock reasoned. "It's important."
"In what way?" John had caught up with his friend's long paces now.
"I got a letter. From McGonagall."
"Saying?"
"Not here," Sherlock hissed. John was surprised.
"What is it, Sherlock?" he hissed back.
"It's about the Irene girl," he muttered.
"Ah," John said. They had reached the door to the Ravenclaw common room. Sherlock knocked on it and to John's surprise, the eagle on the knocker spoke.
"What is the capital of Azerbaijan?" it asked sleepily.
"That was a rubbish question," Sherlock scoffed. "Baku."
"I'm tired," the knocker mumbled. "You can enter." The door swung open and the duo stepped inside.
Moonlight shone through the ceiling to floor windows and John looked around the tower. There were bookcases on all the walls and a single statue of a beautiful woman dominated a section of the room. "It's rather nice," he said.
"Oh, don't try to compliment it. You prefer the cosy warmth and homeliness of Gryffindor tower, I bet."
"How do you know that Gryffindor tower is cosier than -"
"I listen. I deduce." Sherlock spun on the spot. "Wait here."
John complied and Sherlock vanished into the shadows. "Lumos," John muttered, pointing his wand towards where Sherlock had gone and seeing a doorway. The dorms, he guessed. Some 2 minutes later Sherlock reemerged, dressed in a blue dressing gown and holding a folded sheaf of parchment.
"This, John . . . this is what I wanted to show you."