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He spent the rest of the afternoon the way he'd spent the morning: playing games with the Weasleys and eating Christmas food. Ron asked him if he'd found anything on Fluffy, and he shook his head, a little guiltily.

Well, he could look tonight, couldn't he?

The idea struck him like lightning while they sat around the fire in the evening, watching Percy chase after Fred and George because they'd stolen his Prefect badge. He sat bolt upright and stared into space.

The Restricted Section. That would be exactly where books about gigantic monstrous poly-cerebral dogs would be. Madam Pince couldn't stop him if she couldn't see him….

"Oi, what's up with you?" said Ron.

"Nothing," said Harry. "It's just an idea I've had. I'll tell you about it later."

"Is it about the PS?"

Harry nodded and sank back into his chair.

But he had to do a proper test run before dragging Ron into it. If he could make it to the library and back just once, there was no reason Ron couldn't do it too.

But he had to do the first one alone.

His father's recipe…

"In hopes you may find it useful."

When they went to bed that night, Ron, full of turkey and cake, fell asleep almost as soon as he'd drawn the curtains of his four-poster. But Harry felt wide awake. The second he heard his friend's breathing even out, he slipped out from his blankets and knelt to retrieve the jar he'd stored under his bed; the sludge-like mixture inside glowed green and very slightly transparent in the moonlight.

He'd taped the recipe to the jar, and he reread the notice at the bottom; it said that a tablespoon of the potion, drunk or taken intravenously, would make you invisible for one hour, and that one drop on clothing or glasses or any inanimate object would have the same effect on them. Below this ran a warning in large bold capitals: "DO NOT TAKE MORE THAN 15CCs (1TBSP) IN A 49MIN PERIOD," followed by several exclamation marks in red ink.

The potion itself tasted a bit like fish, but mostly like lemon cleaning solution. Sure clears your sinuses, he thought as it burned his throat on the way down. He'd have to invest in a syringe, if he could sneak it past the Dursleys….

He watched as the effect spread from his core, like vaporisation in Star Trek. First his middle turned, leaving a great big hole you could see through, and then slowly it spread out into his bloodstream, across his chest and his shoulders and down his arms and legs, into the tips of his fingers and toes and the ends of his shoulder-length hair. A drop on his robe and pyjamas and they turned too, the same way: slowly, spreading from the middle.

Harry glanced in the mirror. His hair was the last thing to disappear; for a second it looked like a badly-groomed wig floating in midair (he'd have to remember to shower again). He grinned and twisted his body as far around as was possible while still looking in the mirror, to see if his back had gone. When he was sure he had completely disappeared, he set off for the Restricted Section, anxious to get his hands on some information about giant three-headed dogs….

It didn't quite go as planned. These things never did, of course—that he had encountered Fluffy in the first place should have been enough proof of that—but somehow Harry just hadn't expected the first book he picked up to scream at him like a banshee.

He dropped it hastily, but it went on screaming, one high, unbroken ear-splitting note. He heard footsteps outside, stuffed the book back on the shelf, and ran for it.

He passed Filch in the doorway. The pale, wild eyes looked straight through him; Harry slipped under Filch's outstretched arm and streaked off up the corridor, the book's shrieks still ringing in his ears. After rounding several corners at random, tumbling through half a dozen doors, accidentally finding at least one and possibly two secret passages, backtracking and zigzagging, running up two staircases and down one…

…he came to a sudden halt in front of a tall suit of armour.

It occurred to him, later, that he should either get to know the castle much better or somehow acquire a map before attempting any more of these night-time forays, because once again he had no idea where he was. Suit of armour. What suits of armour did he know of? There was one near the kitchen, but he must be five floors above that; there was one near the third floor corridor, but he would recognise that anywhere….

"You asked me to come directly to you if anyone was wandering around at night, and someone's been in the library—Restricted Section." Harry jumped. Wherever he was, Filch must know a shortcut, because his soft, greasy voice was getting nearer.

To Harry's horror, it was Coach who replied.

"Restricted Section? Really? Oh! Well, they can't have got far. See who it was?"

"No, I didn't see anyone, they might have used the hidden door…"

Harry saw them round the corner. A quick glance at his hands showed him to be still invisible, but he had no idea how long it had been since he'd taken his dose, and anyway it was a narrow corridor and no amount of potion could keep you from being solid when you got bumped into….

A door stood slightly ajar to his left; it was his only hope. As he held his breath and slipped through, he thanked his lucky stars he was so skinny—Potter and Filch didn't even notice, and he made it into the room.

They walked straight past, still talking in low voices. At first Harry was completely invested in listening for their footsteps to die away, so a few seconds went by before he noticed anything about the room he had hidden in.

It looked like a classroom, but it couldn't have been very frequently used. Desks were piled on top of each other, chairs stacked in haphazard piles, and the wastepaper basket stood on its head. A spare classroom, more than likely, or perhaps a storeroom for surplus furniture—but something propped against the wall facing him didn't look as if it belonged there, storeroom or not.

It was a magnificent mirror, as high as the ceiling, with an ornate gold frame, standing on two clawed feet. There was an inscription carved around the top in elegant swirling script: Erised stra ehru oyt ube cafru oyt on wohsi.

He automatically began to reverse the letters in his head; it took him a moment to work over one or two of the tricky bits. Is how…ish ow…I…show…?

Heart's desire. Heart's desire? How could a mirror show your heart's desire? (Well, obviously, by magic.) Did he even have a heart's desire? To find out how to get past that dog, maybe. But could a mirror show something like that?

His panic fading now, Harry moved closer to the mirror. At first he thought that the potion must have worn off in the past two minutes, because his reflection appeared as he approached, perfectly normal and entirely visible. He checked his arm, though, and it was still transparent. (There should be some way of just telling when an hour was past; he had considered bringing a watch, but then he thought that it wouldn't be any good if he didn't turn it invisible, and even less good if he did.)

Harry glanced back at the mirror and started violently.

While he'd been looking away, a whole crowd of people had suddenly materialised in the mirror. The boy in the centre was still him, but on either side of him and behind him stood at least ten others, mostly grown-ups.

He re-read the inscription hastily. Your heart's desire. The reflection in the mirror was only a projection of his deepest wishes, it wasn't real.

But for something that wasn't real, the image was really, really clear. He reached behind him, watching his reflection carefully the while—had there in fact been anyone there he would have touched the robes of an older woman with black robes. She looked vaguely familiar….

"Grandmother!" Harry gasped.

It was, indeed, his grandmother. Her hair was darker than he remembered, and for once her old face was creased in a sort of smile. It made her look younger, even a little prettier.

He quickly scanned the crowd for her husband, but apparently his heart's desire involved neither Tobias nor any of the Dursleys. In fact, apart from his grandmother, he didn't recognise anyone there, though all of them seemed to know him.

His grandmother moved aside after a moment, and a young woman took her place. She smiled at Harry, too, and waved. She was a very pretty woman, with dark red hair, and her eyes—her eyes are just like mine, thought Harry suddenly, moving closer to get a better look. Bright green, exactly the same shape. Harry noticed that she was crying—smiling, but crying at the same time.

A hand took her shoulder and drew her into an embrace. She didn't appear to be able to look away; she leaned back into the arms of a man much taller than she was, her eyes still glued to Harry. The man looked at Harry, too; he nodded slightly and blinked back tears of his own, like a very stern adult overcome with pride and approval.

Like Grandmother, the man wore black, and like Grandmother he had deep, premature lines on his face. In fact, a lot of him was like Grandmother. He had the same skinny frame and gaunt cheeks; the same jewel-black eyes and sallow skin; the same dark hair that looked as though it needed washing often, like Harry's did…

…like Harry's did.

That young woman had Harry's eyes, but this young man had Harry's hair. And his nose…his nose was the same one that Harry had seen and hated in the ordinary mirror every morning for eleven years. And he had those same hollows under the eyes that made him look like he didn't sleep. And that same mouth with the turned-down corners. And those same crooked teeth….

Harry was so close to the mirror now that his nose was nearly touching that of his reflection.

"Mum," he whispered. "Dad."

The woman cried harder and held out her hand toward Harry, which he nearly smashed the mirror trying to reach. The man looked as though he wanted to say something but couldn't find the words; in the end he settled for a smile of his own, the sort that makes you feel special because you know the person doing the smiling rarely smiles.

And Harry looked back over the rest of the crowd and saw other green eyes like his, and other noses like his, even a little old man who looked as though he had Harry's knobbly knees…Harry was looking at his family for the first time in his life.

Of course, he thought, staring hungrily at the crowd of waving people. Heart's desire. Fluffy? Who cared about some stupid three-headed dog? All he wanted, all he had ever wanted in his entire life, was to be with these people in the mirror—his grandmother, who taught him to play Gobstones and protected him against Tobias; uncles and aunts who wouldn't starve or hide or beat him; brothers and sisters and cousins to play with; a home to go to in the summer and a family to miss him and greet him…his eyes strayed to the beautiful redhead, still in her husband's arms. A mother to sing him lullabies and tuck him in at night even when he got too old, and make him take second helpings and worry that he didn't have enough friends. A father to teach him how to ride a broomstick and kill a spider and drive a car, to tell stupid stories about "when I was your age", and tease him about girls.

They looked so close, so real. He could have reached out and touched them. A powerful ache filled him, part joy, part terrible sadness, and part useless, burning rage. He hit the mirror with his fist and went on looking and looking, drinking in the images. The lump in his throat swelled until he felt he could hardly hold back tears any longer. He wished he could stay here forever.

Of course, he couldn't. He looked at the door apprehensively. Potter or Filch or anyone could come through it at any moment, and he realised with horror that the potion had actually worn off—he must have been here ages. He had another dose in his bathrobe pocket, and he would definitely drink it, in just a second. Maybe just five more minutes, he could spend with his family…

But when he looked back at the mirror, the image had disappeared. In fact, even his reflection had disappeared; the shiny surface of the mirror now showed what looked like the Hogwarts Great Hall, full of people.

"What? No! Come back! Hey! This isn't…"

Then he blinked. It looked like a Sorting was going on. On the stool, with the battered black hat on her head, perched a young girl. She had green eyes that shone as bright as stars. With her eager face and red hair, she reminded Harry of the little girl he'd met in King's Cross, the one who'd smiled at him.

"Mum?" It had been weird enough seeing her as an adult; now she sat in front of him, his age, about to be sorted the way he had been barely three months ago. It occurred to Harry that he didn't even know what house his parents had been in. Since he couldn't hear what was being said, he decided to watch and see where she sat.

Young Lily clasped and unclasped her hands as the hat deliberated for what seemed like a long time. Eventually it made some kind of pronouncement. She hopped off the platform, turned left, away from Hufflepuff and Slytherin—passed Ravenclaw—and sat at Gryffindor.

"Yes!" said Harry, grinning. "Now show me my dad."

The next student it showed didn't look anything like Severus Snape; he had black hair, but that was about the only similarity. Harry, with a jump, recognised Coach Potter, eleven years old and without glasses. He swaggered up to the stool, put on the hat for a fraction of a second, and swaggered to the Gryffindor table.

"Stupid mirror, I said show me my dad…."

This time it obliged. Young Severus looked almost exactly like Harry, down to the thinness and unhappiness. He looked unhappier still when he put the hat on; he'd placed it badly and it nearly tipped off his head.

When a much-younger Professor McGonagall righted it, it sat on his head for a moment and then made its pronouncement. Severus got down from the stool, stepped off the platform, turned left, passed Ravenclaw, and seated himself in the place Lily had saved for him at the Gryffindor table. She gave him a hug. He gave her the same sort of smile older Severus in the mirror had given Harry.

"Both Gryffindors," whispered Harry. "Just like me."

A distant noise startled him and he whirled around. He had to find his way back to bed. He had at least to take the Potion; he was standing there like an idiot staring at things that didn't exist, just waiting to get caught. He hastily downed his emergency dose, waited until it had worked, and looked back at the mirror. The image had returned to the one of his family. His parents smiled at him, as if they'd watched the Sorting scene and were pleased that he had too.

"I'll come back," he whispered. "I have to go, but I will come back."

"You could have woken me up," said Ron.

"Don't be cross, Ron, I'm going back and you can come. I want to show you the mirror. I don't know whether you'll be able to see my parents or not but you can see what your heart's desire is."

"Might be fun," said Ron slowly, "but…what good is it, I wonder? I mean, don't most people already know what they want? What's the point of being able to see it? Won't that make it worse?"

"It's…it's just interesting, is all," said Harry.

"Well, shame about not finding out about Fluffy, in any case. That potion's brilliant, no wonder I couldn't find you when I went up to the dorm the other day. You should use it to go to the library, maybe tonight after we see the mirror."

"Now who's channeling Hermione?" Harry snapped. "Don't you get it? Fluffy isn't important. Who cares about some old stone? Why shouldn't Potter get it? I mean, maybe he's supposed to, for all we know. And Hagrid did tell us to cut it out."

Ron's mouth dropped open. "Are you kidding? You're the one who started all this!"

"I know," said Harry. "But I don't care about it anymore. I want to see my parents again."

"Harry…" Just then Fred and George strode in, late, as usual, and the conversation was over.

Harry had taken care to pay attention to his route when he'd gone back to his room the previous night, and he'd traced and retraced his steps a couple times to make sure. That evening he gave Ron a dose of the potion, which made him gag, and tied a piece of string between their wrists so they wouldn't lose each other in the corridors.

"It's here," he whispered when they got to the door. The mirror stood, ornate and beautiful, on its little clawed feet.

"What's that language?" Ron's voice asked.

"It's not a language, it's backward, like Leonardo da Vinci."

"Like who?"

"Come on."

There they were. His mother beamed at the sight of him; his father only nodded, but his eyes brightened a bit.

"Can you see them?" Harry asked.

"No, I just see the classroom."

"Maybe you can't see each other's heart's desires. Here, look in it properly…."

There was a silence, then Ron said, "Wow."

"What do you see?"

"Not my family," said Ron. "I'm alone—but I'm different—I look older—and I'm Head Boy!"

"Really?"

"I'm wearing the badge like Bill used to—and I'm holding the House Cup and the Quidditch Cup—I'm Quidditch Captain too!"

"That's your heart's desire? A bunch of cups and a badge?"

"Oi, watch it! You wouldn't think families were so great if you'd had one all your life. Well…it's certainly interesting, you were right about that. It kind of gives me the creeps, though. Let's get out of here."

"I want another look."

"Harry, you said you spent an hour looking at it yesterday."

"Well, I want to look again."

"Harry!"

"What?"

"I—think we should go."

"You don't get it; you see your family all the time."

"Harry, I don't think this is a good thing, you've been acting really weird…"

"Wanting to have a real family is acting weird?"

"It's not a real family."

"Well, they're the next best thing!"

"It's not even a they, it's just a picture! You're just going to sit here all night staring at a shiny surface?"

"I don't get what's so wrong with…"

A sudden noise in the corridor put an end to their discussion. They hadn't realised how loudly they'd been talking. They stood stock still as the luminous eyes of Filch's cat Mrs Norris rounded the corner. She seemed to look right at them and Harry wondered if cats could see through invisibility potions. After about an age, she turned and left.

"This isn't safe, Harry, she might have gone for Filch, I bet she heard us. Come on."

And Ron pulled Harry out of the room.

"Harry, don't go back tonight," Ron said the next afternoon as Harry stared dreamily out the window.

"What?"

"To the mirror. You've been weird all day and it's like you don't want to do anything. I think you should stay away. And what if someone catches you? I bet that potion's got an antidote or a counterspell or something, what if Coach or Filch runs into you or you knock something over?"

"Calm down, Hermione," said Harry. "Nothing's going to happen."

"I'm serious, Harry. Don't go back."

He ran all the way to the disused classroom that evening, making more noise than was probably wise, but he didn't meet anyone.

And there was his mother, looking like she wanted to sweep him up in her arms, and his father almost bursting with pride and love, and his grandmother actually smiling, and another old man with green eyes, probably Lily's father, nodding happily. Harry sank to the floor in sheer bliss, wanting to spend all night just looking and watching. Maybe there would be another scene from the past, like the Sorting; maybe he would be in it this time, he could see himself as a baby. Sleep didn't matter anymore; there were so many possibilities and nothing to stop them.

Except—

"So—back again, Harry?"

Harry felt as though his insides had turned to ice. No one could see him, surely? He'd taken the potion…

"Yes, Harry, you are quite invisible, and no one can see you." Harry turned his head. The man sitting perched on the desk by the wall smiled directly at him. He had long silver hair and a long silver beard and twinkling gold half-moon spectacles. "There are, however," said Headmaster Albus Dumbledore, "certain advantages to…being me."

"I'm sorry, sir, I didn't see you."

"Strange how nearsighted being invisible can make you," said Dumbledore. He slipped off the desk to sit on the floor with Harry. "So, you, like hundreds before you, have discovered the delights of the Mirror of Erised."

"Is that what it's called, sir?"

"You worked out what it does very quickly."

"Yes, sir. I read a book about Leonardo da Vinci once, he was a Muggle who invented all sorts of things, and he used to write backwards, except he used to write the letters backward too. 'I show not your face but your heart's desire.' It showed me my family. It showed Ron winning some award. I wasn't exactly sure what 'heart's desire' meant, sir, but when I see my family I know: it's the one thing I want more than anything in the world."

"That's what Ronald saw as well, Harry. The one thing he wanted more than anything in the world."

"He can't feel about a silly Quidditch cup the way I feel about my parents, sir."

"Harry, you feel alone; he feels overwhelmed, overshadowed by his brothers all his life. A Quidditch cup may seem silly to you, but your desire for family would seem silly to someone without a home or food to eat…." He checked himself, and then his eyes twinkled a little. "Forgive me, Harry. One of the signs of growing old is that you begin to realise the truth in all those silly platitudes."

"What's the mirror for, sir?"

"For, Harry? Nothing."

"Nothing? What do you mean?"

"This mirror shows us the deepest desire of our hearts, but it does not give us either knowledge or truth. In that sense, it is useless. A party trick or a cruel prank: who can tell? It will be moved to a new home tomorrow, Harry, and I must ask you not to go looking for it."

"But sir…sir…"

Dumbledore waited. Harry struggled with his words and tried not to look at the mirror; out of the corner of his eye he could see his mother and father watching the conversation.

"Sir," said Harry, "please. You don't, you couldn't understand. It's like…like pictures. Photographs. People have pictures of people they've lost. You keep mementoes. They're useless, aren't they? No knowledge, no truth. But everybody understands…you've got to have something. I don't have anything. I don't even have pictures. I never saw their faces before…"

Dumbledore nodded, his face very serious and a little sad. "I understand better than you might think. But it's not just a picture, is it? You see yourself with them. It's another reality inside that mirror, another reality exactly attuned to resonate with the deepest longings of your heart. A reality so real you could almost touch it. And it's that almost, Harry, that has driven people mad.

"There was once a man, Harry, who looked into the mirror and, like you, saw something impossible, something that broke his heart, something that entranced him so he couldn't bear to look away from it. Every time he looked his desire grew. He looked too long, and slowly, very slowly, he lost his grip on the difference between what he saw and what was, between possible and impossible. He secluded himself from the outside, lived alone, and eventually went mad with grief and wishing.

"It does not do, Harry, to dwell on dreams and forget to live."

Harry nodded, then realised that he was invisible and said "Yes, sir."

"Good lad. Now, I think you should get off to bed. Make sure to check your dosage; it is always inconvenient to become suddenly visible in a tight corner. And if I were you, I should invest in a stopwatch with an alarm."

Harry stood up. "Sir—Professor Dumbledore?"

"Yes?"

"The man who went mad…did you know him?"

"Yes."

"Was he you?"

Dumbledore grinned. "No, Harry. I'm only a bit mad, you know."

"What happened to him? Is he still alive?"

"If you can call it that. He lives now entirely in the past, his mind an agony of redoubled loss and wishes half-achieved."

"Did…you ever look in the mirror, sir? What did you see?"

"I?" Dumbledore stood up and brushed off his robes. "I saw myself holding a pair of thick, woollen socks."

"Socks?"

"One can never have enough socks. Another Christmas has come and gone and I didn't get a single pair. People will insist on giving me books."

Harry saw a flicker of some kind of deep emotion on the old man's face. It struck him that perhaps Dumbledore was not being quite truthful, but then, it had been a very personal question.

He turned away from the headmaster and walked up to the mirror. His mother bent down as he approached, and he kissed the mirror where her cheek was. "Goodbye," he said. "I'll always miss you."

Lily buried her face in her husband's robes and Severus, putting his arm around her, looked into Harry's eyes and nodded. Harry straightened up, squared his shoulders, and, without looking back, went to bed.