After lunch – Hermione was surprisingly hungry – everyone who was doing Care of Magical Creatures went out onto the grounds.

Ron had said he'd be doing his Muggle Studies homework, but then Neville had pointed out that he had Muggle Studies later that afternoon, and Ron had agreed to wait until after the last period of the day to do his homework so Neville had someone to do it with.

It was all a lot more complicated than when everyone had the same timetable, but the way it all worked out was that Harry, Dean and Hermione all joined the rest of the Gryffindors and Slytherins doing Creatures on a big lawn not far from the lake.

Professor Kettleburn came striding up as the bell rang, made tinny by the distance to the castle, and beamed. "All here? Excellent!"

He clapped his hands together, one of them a normal arm and the other a kind of gears-and-cogs artificial arm that clearly used magic to function. "First thing to warn you is that all magical creatures have a danger rating, from one-X to five-X. These ratings do not mean that a creature will hurt you – it is quite possible to work safely with many of the five-X creatures without suffering any sort of harm. But you should always be careful – a higher X rating shows how likely it is that you will be the one hurt by your mistakes!"

The idea seemed quite entertaining to him.

"Now, who can name an example of a one-X creature. Yes, miss Brown?"

"A Flobberworm?" Lavender asked, sounding quite disapproving of the animal in question.

"Ah, yes, the humble Flobberworm!" Professor Kettleburn agreed. "The very humble Flobberworm, it must be admitted! They don't have the glamour of most magical creatures, but Flobberworms serve very important roles in the development of potions and in finding out if people have the talent for taking care of magical creatures."

He smiled broadly. "Now, two years ago, your first lesson would have been on taking care of Flobberworms – but instead, I think perhaps we should have something a little more impressive."

The Creatures professor whistled sharply, and a pair of big black wings spread at the other side of the lake.

Even though everyone had seen Nora before, there were still gasps as she took off and flew low over the waters of the Black Lake. Ripples curved away to either side of her racing form, created by the wind from her body and from the tips of her wings, and she pulled up into a flare before landing with a thump in front of them all.

"Hello!" she waved, then paused and sniffed – and prowled forwards a little, inspecting Harry.

"You got bigger!" she announced. "When did that happen?"

"It was while I was on holiday," Harry explained.

"You're still small," Nora judged. "Will you grow bigger again?"

"Probably," Harry guessed. "I'm not really sure, to be honest."

"Do you know, that's more impressive than I remembered," Professor Kettleburn said. "Can you do that with any animal, or is it just dragons?"

"I don't even know if it's all dragons," Harry admitted. "So far it's only worked with Nora."

"How peculiar," the Professor mused, then gave Nora a scratch on the top of her eye ridge. "Now! What I want everyone to do is to take notes on Nora here – write down important details, do a sketch, anything you like. Remember to think about what's worth knowing!"

He laughed. "Much more exciting than doing it on Flobberworms, am I right?"

The only observation reports of a magical creature Harry had seen were the ones in the travelogues he'd read (like Around Africa By Broom) and the one for him that Charlie Weasley had done a couple of years ago.

He certainly couldn't draw as well as Dean could, so his sketch of Nora was a bit rudimentary, but apart from that he tried to point out details of things like her wings (where he could mention words like 'Alar phalange', the technical term for the finger-struts that supported a wing) and the identifying features of a Norwegian Ridgeback.

After checking his copy of The Atlas of Beasts and Creatures, Harry also dedicated about half his report to behaviour. He did stick to the things that happened during the lesson, but there was still quite a lot to talk about – in particular how different Nora's reactions were to how a normal Norwegian Ridgeback was supposed to act (i.e. touchy, territorial and quick to anger).

There was a bit about halfway through when Draco asked if Nora was actually a dragon, but Professor Kettleburn laughed and replied that he could ask her to breathe fire if he really wanted to know, adding that he'd been set on fire by her about a dozen times and one of those hadn't even involved a flame-freezing charm.

He said it was a good way to warm up his old bones on a cold and frosty morning.

Care of Magical Creatures was followed by Runes, which was the last new subject on Harry's list. It was sort of nice they were all on the same day, letting him get a good look at them, and he and Hermione met Ron outside a classroom on the second floor.

"...what I've heard is that we're doing star charts and stuff," Ron was telling Justin Finch-Fletchley and Ernie Macmillan. "I'm not sure if that means we'll have to learn how to identify individual stars, though, or just the patterns."

"Like if it's a big giant star?" Justin asked. "What are they called… red giants?"

"Yeah, but it's weird because stars have to be big and close to show up big," Ron replied. "Sirius isn't especially big, it's just – oh, hey guys."

"Afternoon," Harry agreed, waving.

"How was Creatures?" Ron added. "What did you start on? Quintapeds?"

"Don't be silly, they wouldn't put a five-X creature in the first class," Ernie replied. "Present company excepted, of course, Harry."

"Actually, unlike Harry, Nora is a dragon that's in Fantastic Beasts," Hermione told him. "So you could say that Harry isn't a five-X creature at all, but Nora certainly is."

"You mean you studied Nora?" Ron asked, and sniggered. "Bet that wasn't too hard for Harry, he can speak to her."

"Everyone can speak to Nora," Harry protested. "If you use simple words, she might even understand you, though only Hagrid really knows enough Dragonish to understand any of her answers."

"Yeah, mate, but by that definition I can speak to my griffin," Ron pointed out. "Anyway, what do you think we'll be doing in the first lesson?"

"I expect it'll be Futhark," Hermione said. "That's certainly what our textbook is about."

Professor Vector had been thin and fair, with a little-or-no-nonsense attitude to teaching her subject, while Professor Kettleburn was full of enthusiasm and missing a slightly alarming number of the limbs he'd been born with. Professor Babbling was different again, a pleasant-looking witch who looked younger than Sirius (which probably meant she was about Sirius' age) and who welcomed them all into her classroom individually.

Once they were sat down, she walked up to the front and chalked a few words on the board. They were in the slightly spiky Futhark rune alphabet, instead of English, and as soon as she was done she turned around to the class.

"Runes," she began, "are languages. To be more specific they are written languages, and specifically languages which, for one reason or another, were and are used for the crafting of magical effects that do not require a wand."

She smiled. "No doubt that all sounds very impressive, but I'm afraid that actually doing those things is usually a lot more bother than just using a wand to do the same thing. We do some rune crafting at Hogwarts, but it's mostly going to be things you could do with your wand much more quickly. Though some of my students do make useful things to keep around in future, and I hope you will be more of the same."

As a demonstration piece, Professor Babbling brought out a little device that looked a lot like a desk fan, if a desk fan had been designed by someone who was only familiar with windmills.

"The rune sequence is broken, like this," she explained, putting her finger on one of the bits of what looked like stone. "But if I push this in here..."

The stone slid into place, and there was a rising whirr as the blades spun up to speed.

Susan Bones put her hand up. "Miss, isn't that something you could do with a wand?"

"You could indeed!" Professor Babbling agreed. "But I doubt you could make one that still works after seven hundred and forty years without needing the charms renewed."

"Blimey," Ron breathed.

"That is the reason why anyone does rune crafting at all," Professor Babbling told them brightly, deactivating the runic windmill and putting it away in a drawer. "Wanded magic is usually quicker, more versatile, and quite convenient enough even before you consider how it is much easier for most people to do. But a runic object made correctly will, quite simply, never fail until the runes themselves have been damaged."

Harry could definitely see how that would be useful.

Professor Babbling went through the basic qualities of magic runes for the next few minutes, such as how runes were both stronger and more durable (and thus more worth the effort) the more permanent and better marked they were, and how you could make it so that a set of runes didn't start "working" until you were ready for them.

At the conclusion of that lecture, she looked around at them all.

"Sadly, to be able to do any of that, you first need to understand how to both write and read runes," she told them. "The runic language we cover to OWL level is called Futhark, and one reason we use it is that it is composed entirely of straight lines – which are, naturally, much easier to write exactly than curves..."

Runes felt a lot more complicated than English, though some of it did remind Harry of the magical languages in other books he'd read. Like how usually runes had a meaning by themselves, instead of only really meaning anything when they were arranged into words.

(Well, Harry supposed that an A had a meaning, and a B had a meaning – or, at Hogwarts, an O had a meaning and an E had a meaning – but most runes were like that and had that meaning all the time.)

It sounded like they were only beginning to explore the subject, which was great, and Professor Babbling's homework for them was to write out a runic alphabet with at least one meaning for each of the runes.

Ron said that sounded like copying out of the rune dictionary, but Harry didn't mind – it was a nice simple start, and it might be helpful to have some parchment like that around. Though he was hoping they'd get to how to do magic things with runes eventually, it might be a nice summer project.

Though perhaps not this summer.

That evening, while Neville was discussing Muggle Studies with Ron to see if there were any differences between their lessons – Neville had found to his surprise that Blaise was one of the Slytherins doing it, apparently – Harry finally finished the last of his Transfiguration homework.

He'd made it a bit longer than usual by talking about Sirius and Percy, as well, as examples of Animagi and how it could be quite easy or quite hard to see the thing which an Animagus carried over from their human body to their animal one. (Percy's flash of red was an easy one, but Sirius had been kind of tricky to work out. It had only been after seeing both of Sirius' forms several times that Harry had realized it was his shaggy mane of hair that Padfoot sported, not just a normal dog's fur.)

As he put his quill down and stoppered his ink bottle, Seamus and Parvati came over – accompanied by about half the Gryffindor first years, including the Barloses.

"Hey, um, Harry?" Seamus began. "You got a moment?"

"Sure," Harry agreed. "I was just finished anyway."

"Told you," Parvati said smugly.

"Hey..." Seamus complained. "Anyway, some of the firsties were wondering something, and they asked us, and someone said you might know."

"We had our first Defence lesson," Mopsy explained. "And there was someone called Professor Lupin teaching it."

"But we were kind of looking forward to a wolf Professor," Cottontail added.

"There's a wolf in second year Hufflepuff, I heard," added one of the first-year boys. "My brother said."

Harry frowned, wondering how to explain it.

"You spent some of the summer with Sirius Black, right?" Seamus checked. "He's supposed to know Professor Lupin. What's that about?"

"They're the same person, but he gets kind of embarrassed about it sometimes," Harry said eventually. "I don't think he can talk when he's a wolf, so it would probably be hard to teach the lessons."

"Oh!" Cottontail said, with the air of someone who'd just had a great mystery solved. "That explains it! Thanks!"

"Told you!" someone called from the back.

"Martin, you didn't say that," another girl grumped.

"I did!" Martin replied. "I said he was an Animagus!"

"And don't worry about asking me questions like that yourself," Harry added. "If I'm not busy doing homework, it's no trouble – and if it's important, don't worry about if I'm busy or not."

"Blimey, that's sounding a bit Prefectish," Ron volunteered. "Isn't that their job?"

Dean snorted. "There's, what, six Prefects? There's a lot more than six first-years."

"Yeah, I think anyone who's available should be okay with being asked questions," Harry shrugged. "If it makes things easier, anyway."

"Neat," Seamus declared. "Well, thanks for that."

As they left, Harry looked over to Hermione, and frowned.

"When did you get up this morning?" he asked. "You look tired."

"I got up later than you, Harry," Hermione replied, looking over the columns of numbers for her Arithmancy. "Hmm… do you think the book means a fifty-two card deck, or a fifty-four card deck?"

"One of the other problems says you divide the cards into four sets of thirteen," Harry pointed out. "Hermione, I don't know how you did all those classes today, but don't forget…"

He trailed off. "Maybe you should ask Percy how he dealt with it. He did all twelve subjects as well, and he was a Prefect in fifth-year on top of that."

Hermione looked torn, then sighed.

"I really should, shouldn't I?" she asked. "Okay, I'll do that, but I'll finish my homework first."

"Sure," Harry smiled. "But don't forget, Thursdays are going to be the worst homework days for you this year. Probably for me too..."

"Hey, so I had this idea," Dean said then, attracting Harry's attention. "You know how people don't know what electric stuff does and doesn't work at Hogwarts?"

Harry nodded, remembering a discussion from last year.

"Well, I realized that old film cameras don't actually need any electricity at all – not really," Dean went on. "Just a bright light and a crank, and you can make the bright light with a wand anyway. So you could show silent films here."

"The Wizarding Wireless works too," Harry said. "I wonder if there's a way for wizards to record sound? Or is it all live?"

Neither of them really knew, but it was kind of fun to talk about that sort of thing.

Fred and George had come over halfway through, looking like they wanted to ask Harry about something, but then exchanged a glance and bolted upwards to their rooms.

They'd probably thought of something.

Bright and early the next morning, before breakfast, Harry flew down to Hagrid's hut to say hello.

Hagrid was quite pleased to see him, and so was Fang – apparently more than a year of exposure to Nora and two to Harry had slowly trained the big dog to see dragons as less threatening than before – and he was quite touched to hear that Harry wanted to take a few photographs.

Harry had never actually heard about how you were supposed to behave when you got your photo taken in the wizarding world, partly because Sirius had either never bothered to explain or never learned in the first place, so he was glad to listen when Hagrid gave him a quick rundown. The polite thing to do was to try and be pleased that you were in the photo, and to wave, so that in the photograph your picture was pleased to be there as well.

As for Nora, the whole thing left Nora very excited indeed. She jumped around a lot, actually making it quite hard for Harry to take a photo that had her in it, and after a few minutes Hagrid decided the best thing to do would be to have her sit down and give her a nice scratch while Harry took the photograph.

After breakfast, the morning was mostly taken up with Double Potions.

Harry was curious about whether Aunt Petunia had known Professor Snape (back before he was Professor Snape, that was) but that was just something to be curious about. What was actually important to concentrate on was the potion they were making, which was one of the shrinking potions from the summer homework.

"Anyone who bothered to pay the least bit of attention to their homework over the summer will doubtless have noticed that the Shrinking Solution is a much more versatile potion than the simple measures for reducing size we have covered so far," Professor Snape began, chalking the instructions on the board.

He paused and then turned to the class. "Anyone who was not aware that we would be doing the Shrinking Solution this year should calm down, and take a deep breath. Doubtless you're the type to forget."

Daphne swallowed a giggle.

"You will have noticed that this potion contains a great number of ingredients," Professor Snape went on. "Since the brewing process takes at least half an hour but not more than an hour, doubtless those of you with working brains will realize you should be doing nothing but preparing ingredients until there is around an hour left in the lesson, and only then start brewing. Pay attention to details like this, and do not be so impatient you begin brewing too early; this will be the only time you get this warning."

Harry wrote it down, thinking it was good advice. It was sort of obvious once you'd said it, but maybe it would be less obvious if you didn't do a lot of cooking?

Anyway, there was a long list of ingredients – the ones on the board were the same as the ones in the book – and Harry got to work, discussing briefly with Daphne before starting on the daisy roots and the shrivelfigs.

"Only one rat spleen," Professor Snape instructed. "Your book says two, because it was clearly written by a dunderhead. Do not be a dunderhead."

"Good life advice," Daphne commented. "Usually, anyway."

Harry wondered why Professor Snape didn't just write a Potions textbook himself.

Most of the lesson was a bit of a blur for Harry – cutting roots so they were split into exactly three equal parts to be added at the beginning, middle and end of the process, measuring out what a "dash" of leech juice represented, slicing caterpillars…

The idea was supposed to be that the leech got bigger when it drank blood, the caterpillars got bigger and then metamorphosed into something else entirely and the Shrivelfig got smaller, so those all sensitized the potion to the concept of changing size – along with half a dozen other things – while the rat spleen was included because rats were small animals that could squeeze through very small spaces.

The axolotl gills were to control how the potion didn't just make things shrink but actually made them younger if need be, which was a really strange effect, and the daisies were sort of there as punctuation between different parts of the process.

Professor Snape informed them that they should be done by now, then brought out half-a-dozen frogs and announced that they were going to test some of the potions.

It was quite impressive when Hermione and Neville's potion was fed to one of the frogs and it shrank down into a little tadpole without any legs, especially when the others didn't shrink down nearly as far.

Then the Professor came to the last cauldron he'd picked.

"Mr. Crabbe," he began, looking up. "Mr. Goyle. Tell me, did you actually pay attention to the instructions?"

"Yes, Professor," Gregory muttered, and Vincent nodded silently.

"Interesting," Professor Snape hummed, taking a few drops of the potion out with a ladle. Electric blue liquid cascaded back into the cauldron, and the teacher raised his gaze again. "Are either of you colour-blind?"

"No, Professor," Vincent said this time.

"That would at least explain why you appear not to have noticed your potion is a different colour to all the rest," Snape observed. "Do either of you know what you did wrong?"

Neither boy replied, though Hermione put her hand up.

"Miss Granger," Professor Snape invited. "Do enlighten us."

"If the daisy roots are added all at once, it means the magical effects of the different stages get mixed up together," Hermione said promptly. "To fix it you'd need some dried Billywig sting and a quarter-ounce of deep-sea angler liver, to separate out the qualities by density, and then-"

"Quite," he said, raising a hand. "Most impressive. Well, I suppose I will do my best to demonstrate why you must follow instructions in my class."

Rather than a frog, this time, he took out a glass box which contained a beautiful Red Admiral butterfly.

"A correct potion would cause the butterfly to shrink smoothly back into a caterpillar," he explained, using a spoon to take a droplet of Vincent and Gregory's potion and slip it into the box.

The red-winged butterfly fluttered down, took a sip, caught fire and exploded.

"If you are foolish enough to think that it would be a good idea to drink that, please, don't let me stop you," Professor Snape said into the silence. "And next time, follow the instructions precisely."

"I still don't really get that," Ron admitted, cutting open a thick slab of pita bread. "Why would it catch fire and explode?"

"It's because of the metabolism," Hermione answered. "It's like… well, your food is flammable, right? Bread is, at least."

"Right," Ron agreed, putting some carved chicken into the pita and adding mayonnaise and lettuce. "But probably not once I've put this much mayonnaise on it."

"I don't know," Harry replied, frowning. "In my experience almost anything is flammable if you try hard enough."

Neville sniggered.

"Exactly," Hermione said, waving her hand at Harry for a moment. "It's got energy in it, that's kind of why you eat it."

"I eat it because it's tasty," Ron countered, taking a bite. He chewed meditatively for a few seconds, then swallowed. "No, needs more spice..."

"You eat things because you're hungry," Hermione tried. "For energy. Anyway, the butterfly had energy in it, and normally it uses that energy to do things like… fly. And… mostly fly, really. But the potion made that all really confused."

"Oh, right," Ron nodded. "So it's like how one of those rocket engines Harry got me burns the fuel steadily, but if you mixed it all up randomly you'd probably get something that would explode."

"...is anyone else weirded out when Ron makes analogies like that?" Dean asked. "Because I am."

After lunch they headed up to the Defence classroom, which seemed to be a different room than before, though perhaps that wasn't unusual – there were certainly plenty of unused rooms – and which contained about half of the third-year Gryffindors already, along with one Marauder.

"Afternoon, Mr. Lupin," Neville said, then blushed. "Oh, I mean, Professor Lupin."

Remus – or Professor Lupin, Harry decided to think of him as – smiled kindly. "It's quite understandable, Mr. Longbottom. Or Neville."

Neville sat down, still looking quite embarrassed.

A few other students filed in, and Harry frowned. It looked like they only had Gryffindor in the Defence lessons this year, which sounded like it would be a lot more work for Professor Lupin than it could have been.

Maybe he was going to do the same thing Hermione was doing?

Harry shook the thought away as Professor Lupin stood up.

"We're going to be doing a practical lesson today," he said. "I would like your help in dealing with a problem in the staff room."

Dean put his hand up.

"Dean?" Professor Lupin invited.

"Um… Professor, are we your first class this year?" Dean asked. "Because if you'd like our help with a problem, how is it still a problem if you've already had other lessons?"

"A very good question," Professor Lupin agreed. "You're the first to ask that. There was a problem in the staff room, but after Professor McGonagall and I dealt with it I realized it would make a good lesson. Follow me, please."

The staff room was surprisingly close to their Defence classroom, and a minute or so later Professor Lupin was waving them all in.

"Who here has heard of a Boggart?" he asked, shutting the door once everyone was in and their bags were in the corner, and a few hands went up. Harry had vaguely heard of them, but he didn't think it counted, and Professor Lupin called on Parvati Patil first.

"They're a kind of spirit," she said. "Quite common hiding in out-of-the-way places in old Wizarding homes."

"Quite correct," Remus agreed. "Neville?"

"Grandmother told me that you should never face a Boggart alone," Neville contributed.

"Also correct," Remus said with a smile. "A Boggart is a spirit, as Parvati says, and it is a shape changer. When someone sees it, it will turn into whatever happens to be their greatest fear."

Ron muttered something about spiders.

"Fortunately, there are several ways in which a Boggart can be countered," Remus went on. "Does anybody know what they might be?"

Sally-Anne Perks suggested that you could just not look at it, asking if that was why people hid under the covers, and Remus laughed before agreeing that it was a good last resort – though he cautioned that once a Boggart had changed it wouldn't go back to being whatever a Boggart was when "resting" if the person it was targeting was still scared.

Lavender Brown supplied that with more than one person they could try to confuse it, and Hermione said there was a spell you could use to disrupt a Boggart, both of which Remus agreed with.

"Very good," he told them all. "Yes, the spell is Riddikulus, and the wand movement is like so."

At his prompting, everyone got out their wands. Harry tried the wand movement first in his paw, and then in his tail, and after a few minutes he was fairly sure he'd got it.

"Since a Boggart tries to cause fear," Remus resumed, "our greatest weapon against it is laughter. To laugh at something is to be less afraid of it, you see, and the Riddikulus charm allows you to force a Boggart's shape to change into something you find amusing."

He smiled. "Of course, a confused Boggart might turn into something amusing by mistake. If one tries to turn into a headless corpse and a flesh-eating slug at the same time, you might end up with a Boggart trying to scare two people at once by turning into half a slug. But the Riddikulus charm lets you force it – though you must concentrate on the change you want to make."

"So..." Ron began. "So if I got a spider, then I could try and make it change into a clown?"

"Oh, great," Fay Dunbar groaned. "Now a spider clown is going to be my Boggart."

"I think that would be too large a change, Ron," Remus told him. "The charm is easier if the change you're making is small but funny."

He paused. "Though I must ask if anyone thinks that their worst fear may be particularly dangerous. If you think it is, you may come over and speak to me privately at some point in the next few minutes. Harry, if you would?"

Harry followed Remus into the corner of the staff room, a little unsure.

"Harry, I have to ask you," Remus began. "Do you think your greatest fear is Lord Voldemort?"

"Well… no," Harry replied, shaking his head. "The last thing he did that I know of was implode, and I think there's also some bits of his soul somewhere, but apart from that… no. It might be a troll?"

Remus nodded. "I can see why that might be the case."

Two or three of the Gryffindors went over to talk privately, and Professor Lupin discussed it with them. Harry didn't listen, instead watching the Boggart-wardrobe as it rattled, and wondered how to make a troll funnier.

Perhaps if you shone a light at it and it froze in place?

Eventually, though, everyone was ready and had their wands out.

"Neville, I think we'll go with you first," Remus decided. "Are you ready?"

Neville nodded, a little hesitantly, then nodded a second time.

"Alohomora!" Remus incanted, sending a jet of light at the wardrobe. The door burst open, and out came a gaunt-looking woman with thick, shining dark hair and heavily hooded eyes. She looked familiar, like Harry had seen her before.

In the quiet, Harry just about heard Remus quietly say "Oh, bugger".

"YOU!" Neville bellowed, then turned and darted for his bag.

Startled enough he wasn't sure how to react, Harry watched as Neville reached into his bag, pulled out an iron bar about two feet long, and swung it at the woman like a cricket bat.

"Riddikulus!" Remus snapped, and when the bar connected there was a loud BONG sound. Little birds circled the woman's head, and she wobbled around a bit before falling straight over backwards.

"Blimey, Neville, what was that?!" Dean demanded.

"That's – Bellatrix Lestrange!" Neville replied, panting. "I… wait..."

He blinked, stepping back. "No, it's a Boggart, isn't it?"

"That was a hell of a reaction," Seamus said.

The Boggart cracked, changing shape, and a bloodstained, bandaged mummy was walking slowly towards Parvati.

Having lots of people around might have been helpful because it was confusing the Boggart, but it was also kind of confusing for Harry as well. One minute the Boggart was a mummy that tripped over its own bandages, as Parvati cast her own Riddikulus, and then the next minute it was a giant bloodstained eye.

"Riddikulus," Fay called, and the eye suddenly had a big eyepatch on it. Then it turned into a rat, which Sally-Anne Perks stopped with another Riddikulus charm that made it chase its own tail in a circle.

"Not really sure how that one's funny," Ron muttered.

"Probably funny to her," Dean replied.

"Ron!" Remus instructed, and Ron took a step forwards.

The Boggart changed with another crack, this time into a giant spider over a foot long with big bristly legs, and Ron swallowed visibly before raising his wand.

"Riddikulus," he called, and the spell sparked a little before fizzling. "Riddikulus!"

The second one worked, and instead of bristly insect-like-legs the spider suddenly had eight little hoofed legs. It made a clopaclopaclopa sound as it tried to walk, tripping over because the new legs couldn't move the same way, and Remus sent Dean forwards to replace Ron.

"My first idea wasn't very funny," Ron admitted, as the Boggart focused on Dean instead. "Maybe that's why it didn't work?"

The Boggart turned into a disembodied hand, which flipped over and began moving a lot like a spider or a crab. Dean cast the same charm everyone else had been casting, which caught the hand in a mousetrap, and then it was Harry's turn to be called forwards.

The sudden transformation surprised Harry as much as anyone else. One moment the Boggart was a hand in a mousetrap, then the next it ballooned outwards into something so big that some people yelped as they were pushed against walls and Harry had to scramble backwards.

It was hidden in a cloud of smoke for a moment, and then the cloud faded away to reveal a great big dragon. Much larger than Harry was and even a bit bigger than Nora, big enough that it barely fit in the staff room even curled up, and with red scales so deep they were almost black.

It was sitting on a hoard of gold and gems, swords and leatherbound books, and pitiless green eyes regarded Harry for a moment before the big dragon growled. Smoke boiled up towards the ceiling from between long fangs – smoke touched by a hint of flame at the back of its throat – and it shifted slightly, all four paws clutching tightly onto parts of the hoard.

There was a Remembrall in the pile, and a little griffin statue, and a fine clockwork watch – all three of them underneath the dragon's nearest paw.

Suddenly feeling sick, Harry looked up to the other dragon's forehead, and he saw a lightning-bolt scar there.

"Mine," the dragon – the other Harry – growled, wings flexing slightly, and a possessive light shone in its eyes. "All mine."

For a long moment, Harry didn't know how to react. It wasn't that he didn't have any ideas what to do – he wanted to shout at the other him, say this wasn't him, that it was wrong. He wanted to attack, to stop it… to somehow show that this was wrong…

This other him was making him feel small-

-and that was what made him realize what he had to do.

"Riddikulus!" Harry shouted, pointing his wand at his double's eyes in case it mattered, and with a sudden whoosh the big red-black dragon vanished.

Mostly.

What was left was a dragon about six inches long, sitting on top of a pile of chocolate biscuits.

"Mine!" it declared again, but this time it was much squeakier.

Seamus snorted, and the Boggart turned its attention to him and transformed into a banshee. Harry gratefully stepped back, and Remus clapped him on the wing shoulder as Seamus made the banshee-Boggart lose her voice.

"Good work, Harry," the Marauder told him. "That's a more grown-up sort of fear, and you clearly didn't expect it. That's why a Boggart is still dangerous to even a trained witch or wizard working alone."

Harry nodded, swallowing.

He looked back to the middle of the room as Hermione stepped forwards, and the Boggart changed into Professor McGonagall.

"I never thought I would have to say this, Miss Granger," the Boggart-McGonagall said primly. "But, since you have failed all your exams, you will be expelled from Hogwarts-"

"Riddikulus!" Hermione yelped, sounding desperate.

McGonagall went silent for a moment. "Issmay angergray, oday otnay indfay isthay usingay!"

If Hermione's giggle was a bit hysterical, Harry was hardly going to mention it, and then Neville moved past her with his wand held ready.

The Boggart changed back into Bellatrix Lestrange, and less than a second later Neville cast his spell. "Riddikulus!"

At first Harry didn't even notice the change, until the Boggart-Bellatrix swept her arm up to cast a spell with a delighted cackle. Then he saw that her wand had been replaced with a large carrot, and once he noticed that he spotted more carrots spilling out of her sleeves.

The sight was absurd enough that about half the class started to laugh, and the Boggart trembled for a moment before exploding into a cloud of smoke.

"Well done, everyone!" Remus announced, as the smoke cleared. "Boggarts often form in wizarding dwellings, sort of condensing out of the ambient negativity, so you may well need to deal with more than one in your adult life. Always remember to have someone with you, and remember the Riddikulus charm, and you will all do well."

Harry did feel pleased, but he also felt a bit like he had after discovering the Mirror of Erised – like he'd found out something deep about himself, something he hadn't thought about until that moment.

He probably wasn't the only one. Though everyone had also found out that Neville's first reaction to seeing a dark witch was to try and club them with the metal bar he used to train his arm strength.

The one that really puzzled him was Dean's Boggart, until Dean explained that he'd watched a movie about 'the Addams family' at Christmas back in 1991 and the hand thing in the film had really freaked him out.

AN:

So that's Creatures with the Bionic Professor (yes, he canonically has mecha-limbs) and Runes.

Then Potions with a Snape turned all the way down to "snide", and the Boggart.

Really, between the Mirror of Erised, the ability to gain an Animagus form, and the Boggart, Hogwarts is all about personal revelations.