Over the next week, more people followed Harry's example and ate meals with friends from different houses. Apparently, just because Harry had been nearly assassinated when he was a baby, this made him famous and everything he did automatically 'cool'. Harry tried to ignore it and just enjoy being with his friends, but he didn't mind having started the 'being friends with people who aren't from your own house' trend.

At Monday lunchtime, when Harry joined Ben at the Gryffindor table, Percy Weasley took the opportunity to go and sit with his cousin Icicle among the Slytherins, calling over his shoulder to his younger brothers to 'Be good!' At dinner, when Ben went to sit with his Ravenclaw friends again, a very pretty fifth-year Ravenclaw girl came to sit with Percy.

The disadvantage of all this swapping about was that if Harry wasn't with the other Slytherins, he didn't always see where they went as they headed out to a lesson. Nobody found it easy to navigate around Hogwarts Castle anyway, but at least most of the Slytherins had wizarding parents who had told them what to expect, whereas Ben and Guinevere's parents hadn't gone to Hogwarts (in fact, from the stories Guinevere told them, Barnabas Greenbloom had moved around a lot throughout his life and had old friends and old enemies in almost every country of the world), Hermione and Harry had been brought up by Muggles, and Hiccup's grandfather hadn't told him or Fishlegs anything very specific about what to expect from the castle.

Even Ben, who had navigated a dragon from Germany to Pakistan, found Hogwarts Castle confusing without a map. 'There's a rat I know who makes the most amazing maps,' he said. 'Maybe we could ask him to come here for a holiday.'

When a lesson took place directly after breakfast or lunch, they could follow a teacher there, but otherwise they were left to wander and ask for directions from any passers-by, whether older pupils (Harry soon learned not to ask Fred and George Weasley), the ghosts, or the grumpy caretaker, Argus Filch. There had been a Potions lesson timetabled on Tuesday mid-morning, but Harry and Ben had been lost in the dungeons corridors for an hour without finding the laboratory, until it was time for Herbology. So it wasn't until Friday morning, when they had a Potions lesson first thing, that they could follow Professor Snape to the right place.

Breakfast was half finished when Hiccup came in, with Toothless fluttering behind him, and joined Ben, Harry and Twigleg (at the Slytherin table this time). 'I saw Hagrid, while I was out,' he announced. 'Ben, he says do you want to come to tea this afternoon, around three?'

'And Harry?' asked Ben.

'I don't know,' Hiccup admitted. 'He only said Ben and Toothless and me. But I'm sure it's okay for Harry and Twigleg and Fishlegs to come too. Probably,' he admitted. 'Grown-ups can be a bit weird about who it's okay to be friends with, but…'

But there wasn't time to discuss it further, as everyone was getting up to go out to classes. Ben and Harry only had time to call, 'See you later!' before they hurried off, trying not to lose sight of the end of Professor Snape's long black robe.

Professor Snape called the register, and made a tick without comment when Ben called, 'Present!' but paused when Harry did the same.

'Ah, yes,' he said. 'The great Mr Potter decided to grace us with his presence for once, did he?'

'That's not fair!' protested Ben. 'We just couldn't find the class, on Tuesday. I missed it too, so why are you picking on Harry?'

'I do not recall asking you to speak, Mr Greenbloom. That will be five house points each from Slytherin and Gryffindor for your absence last time, and an additional five from Gryffindor for interrupting.'

Draco Malfoy, Vincent Crabbe and Gregory Goyle, who had been enjoying laughing at Harry, now turned to glare accusingly at him.

'So, Potter, as you clearly do not believe you need to attend every lesson, doubtless you are already an expert. What would I get if I added powdered root of asphodel to an infusion of wormwood?'

Harry blinked. Vincent Crabbe and Gregory Goyle were now roaring with laughter, even though from what he had seen of them in Herbology, Harry was sure they couldn't have answered this either. He had Herbology next, with the Ravenclaws – Twigleg was sure to know the answer to this, even if Hermione didn't.

'Where would you look if I told you to find me a bezoar?'

Harry's mind raced. 'Uh – in the supplies cupboard under "B"?' he managed.

'Very funny,' sneered Professor Snape. 'What is the difference between monkshood and wolfsbane?'

This one Harry did know, as Twigleg had been telling him about them as they worked together in the last Herbology lesson. 'They're both species of aconite, but monkshood, Aconitum napellus, has indigo flowers, while wolfsbane, Aconitum vulparia, has white or straw-yellow flowers, except that Aconitum lycoctonum is also called wolfsbane or northern wolfsbane, and that has purple flowers, and it's the only one that can be pollinated by butterflies as well as bumblebees.'

'Oh, quite the expert, aren't we?' said Professor Snape sarcastically. 'And which alleged "wolfsbane" would you use to treat lycanthropy?'

The only treatment programme for lycanthropy that Harry had heard of (from Guinevere this time) involved a vegetarian diet and a special amulet, but he was fed up with being interrogated. 'How should I know? You're supposed to be the teacher!' he burst out.

For a moment, Professor Snape looked as though he wanted to throw something, preferably Harry. Then he paused, as if thinking something over. 'Potter,' he said more gently, 'did your family allow you to read and study in the summer holidays? I know some Muggles don't take kindly to having a wizard in the family.'

Harry knew that Professor Snape was thinking back to what he had seen of Uncle Vernon and Aunt Petunia. It would be so easy to get out of trouble here by lying, claiming that they hid his books and kept him locked in the cupboard all the time (and make it sound as if it was a tiny box where he was kept chained and blindfolded, like that man who got kept as a hostage in Beirut). But then again, Professor Snape could quite possibly read minds.

'They mostly just ignore me,' he said. 'And I ignore them. And I have been reading, only I couldn't remember what the book said about asphodels and bezoars, and I only knew about aconites because a friend was telling me about them. I'm sorry, sir,' he managed.

Professor Snape looked very slightly mollified. 'Would that be the homunculus or the girl with teeth like a rabbit?' he asked.

'It was Twigleg. But Hermione's good, too. If she was in this class, she'd have been able to give you all the answers straight off.'

'Yes, Miss Granger answered everything perfectly. She also managed to find this classroom the first time round. However, since your friends will not always be here to help you, and, as you were kind enough to remind me, I am the teacher, let me inform you that there are over one hundred and twenty species of aconite, several of which are commonly known as both wolfsbane and monkshood. For homework, in addition to the whole class's assignment, you will write me two sides of parchment on the differences between Aconitum anthora, Aconitum carmichaelii, Aconitum ferox, Aconitum fischeri, Aconitum flavum, Aconitum heterophyllum, Aconitum coreanum, Aconitum lycoctonum, Aconitum soongaricum, Aconitum uncinatum, Aconitum violaceum, and Aconitum vulparia, to be handed in next Tuesday.'

Harry wrote these down while Ben fetched the ingredients they would need for the first practical lesson, a potion to cure boils. Thankfully, Ben was methodical and careful at Potions, and hard as Professor Snape tried, he couldn't find anything to criticise when he passed their table. Before they could complete the potion, however, Neville Longbottom's toad leapt up from the table where he and Ron Weasley were working, knocked into Harry and Ben's cauldron and sent the potion hissing and bubbling all over the floor.

'Onto your chairs!' Professor Snape shouted.

Everyone obeyed except Neville, still trying to recapture his toad. He slipped and fell over in the smoking green mixture, and struggled to his feet, sobbing with pain and fright as the potion bubbled his skin into a mass of boils, and desperately calling out, 'Trevor!'

Harry caught a glimpse of movement of a small, crawling creature heading towards the spreading puddle. He leapt over the edge of the pool of potion and bent down just in time to pick the squirming little amphibian up.

'Very pretty,' growled Professor Snape, vanishing the spilled potion from the floor. 'Save the display of athletics for when you make it onto the Quidditch team. That will be five points each from Gryffindor and Slytherin for disobeying, and another five from Gryffindor for causing the accident by bringing an out-of-control animal to class. Weasley, take Longbottom to the hospital wing; Finnegan, get that animal out of my sight. Longbottom, when you've recovered, report to my office to discuss what your punishment will be.'

Neville went white with terror. 'You've already taken ten house points off him!' Harry shouted. 'And he's hurt! Isn't that enough punishment?'

'Silence, Potter. That remark just cost another five points from Slytherin.'

As Neville Longbottom, Ron Weasley, and Seamus Finnegan holding Trevor the toad all left the room, Professor Snape took out a fresh piece of parchment and wrote out a notice:

ABSOLUTELY NO PETS ARE PERMITTED IN THE POTIONS DUNGEON UNDER ANY CIRCUMSTANCES. UNAUTHORISED ANIMALS WILL BE USED AS EXPERIMENTAL SUBJECTS OR POTIONS INGREDIENTS.

'We've got to warn Hiccup,' Ben said, as they left the lab with notes of the class's homework (an essay on common causes of potions accidents and how to prevent them). 'He takes Toothless everywhere with him, even more than Neville does with Trevor.'

'I know. I've got Herbology next with the Ravenclaws. I'll tell him then.'

In fact, he didn't have the chance to have that conversation, but didn't really need to, as Toothless proved the point for himself. As Professor Sprout led the assembled Slytherin and Ravenclaw first-years down to Greenhouse One, Toothless, who had been draped around Hiccup's shoulders like a scaly green scarf, suddenly rose into the air and darted into Greenhouse Three, which housed the more dangerous plants.

'Toothless! No! Come back!' called Hiccup in Dragonese, running in after him. But it was too late. Toothless had found something interesting in one of the plant-pots, and was digging it up.

'Cover your ears!' called Professor Sprout, clamping her hands firmly over her own. All the students copied her (including Twigleg, sitting on Guinevere's shoulder), but it wasn't enough to block out the sound of a hideous, high-pitched screaming. After a short while the sound became more muted, and Professor Sprout, after looking into Greenhouse Three, nodded to the class that they could uncover their ears.

Hiccup was lying unconscious on the floor, beside a broken plant-pot with a chewed stalk and a few rather hairy green leaves. Toothless, on the other hand, was flying frantically in circles, babbling nonsense: 'Issa purple-spotted three-head Hippogriff! Toothless's t-tail is an oak-tree! Toothless's wings are m-m-melting! Toothless need to chuck-up now! Toothless need to poo-poo now! Toothless can't SEE!' he ended in a wail of terror, collapsing in a frightened green heap in Professor Sprout's arms as he spurted out a jet of foul-smelling gunk from both ends. Then he was silent – but his stomach, and the puddles of vomit and diarrhoea staining Professor Sprout's robes, could still be heard faintly screaming.

'And this is why we no longer use animals to harvest mandrakes,' said Professor Sprout. 'Mr Potter, can you take this dragon to Hagrid, please? And' (she conjured a stretcher) 'Miss Greenbloom and Miss Granger, can you take Mr Haddock to the sickbay?'

'I think he's not the only one we need to take,' said Guinevere. They all noticed that her hand had gone up to her shoulder to steady the body of Twigleg, who was also unconscious.

'Be careful!' said Hermione. 'You mustn't let Twigleg get near the mandrake juice!'

'No, indeed,' said Professor Sprout. 'Can anyone else explain why?'

No-one could – except Hermione. 'Because he's a homunculus!' she explained. 'The mandrake root is used to return people who have been cursed or transfigured to their original state. And a homunculus is created by transfiguring a minibeast such as an insect or spider.'

Guinevere's mouth opened in horror. She checked Hiccup very carefully for any trace of mandrake before lifting him onto the stretcher and laying Twigleg carefully beside his head.

Harry, meanwhile, picked up the filthy, unconscious, and still faintly noisy body of Toothless and carried him across the grounds to Hagrid's wooden cottage on the edge of the Forbidden Forest. As he approached, he could hear a large-sounding dog barking, and Hagrid quieting it before calling, 'Ye're early.'

'Professor Sprout sent me,' Harry explained. 'Hiccup's dragon's been poisoned – he ate a mandrake. Professor Sprout thought you could help.'

'Aye? Poor li'l beas'.' Hagrid went to a cupboard, and took out a large flask of potion. He measured some out into a teaspoon, and dribbled it into Toothless's open mouth. Toothless swallowed, and then went stiller than ever. Harry could feel his body growing cold.

'I think he's dying,' he said.

'Nah, tha's a healin' coma. A unicorn, now, or a centaur, they'd just lie down an' feel sick fer a bit. But dragons, when they're hurt or sick, they go still an' cold, as if they're dead, same as when they're hibernatin'. Yer friend Hiccup must've told yer that, surely?'

'Yes – he told me about that happening to Toothless before, when he was hurt in a fight. He went all still and cold for days, like this, and Hiccup's dad thought Toothless really was dead and was all ready to cremate him, and he only just woke up in time to avoid being burnt to death.'

'Well, he'll be safe from that, here,' said Hagrid reassuringly. 'Never heard of a Gian' Seadragon eatin' mandrakes before, though – they're not exac'ly noted fer bein' vegetarian.'

'Oh, he isn't. I think he was just curious. What was that potion you gave him?'

'Calabar bean an' Manchineel fruit. The unicorns gets into the greenhouse every so offen and tries ter see wha' mandrakes tas'e like, so Professor Snape brews up a batch of antidote when I'm gettin' low. I'll need some more, soon. Where is Hiccup, anyway?'

'He got knocked unconscious by the mandrake screaming,' said Harry. 'So did Twigleg.'

'The homunculus? Poor fellow,' said Hagrid. 'The size mandrakes are at this time o' year, they won't do much ter a human, but fer li'l chaps like Toothless or Twigleg, they can be right nasty. Specially considerin' how sensitive a homunculus's ears are, or a dragon's. Tha's bad.'

'I ought to go back to class,' said Harry. 'I don't know if Hiccup will be here this afternoon, but if he's woken up, I expect he'll probably want to come and see Toothless. But if Twigleg is still ill, probably Ben will be with him. But – did you want me to come?'

'What?' said Hagrid, sounding startled and rather guilty. 'Why wouldn' I?'

'I don't know. But I do know that you told Hiccup to invite Ben, and didn't say anything about me. And – well, Professor Snape told us that everyone would hate us for being Slytherins, and that Gryffindors would in particular. So – do you?'

'O' course not! Do yer think Professor McGonagall hates Professor Snape fer bein' a Slytherin?'

'Maybe not,' said Harry. The two Heads of House seemed to be rivals, but in a mostly friendly way. 'But – did you hope I wouldn't be a Slytherin?'

'I'm just – surprised,' said Hagrid. 'Thought fer sure yer'd be a Gryffindor, like yer mum and dad.'

'Why?' demanded Harry bluntly. 'People don't have to be the same as their parents, do they? Most of my friends don't even have wizards for parents. But Hiccup reckons that if his parents were wizards, his dad would be in Gryffindor, and his mum – well, he says she's away on quests most of the time so he doesn't know her very well, but she's ambitious for something, so maybe she'd be Slytherin. And he's a Ravenclaw.'

'Nothin' wrong with bein' Ravenclaw.'

'But there is with being Slytherin?'

'It was a Slytherin that killed yer mum an' dad, Harry. Nearly all his followers was Slytherins, too.'

'Nearly all isn't the same as all, though, is it?' Harry persisted. 'Are you really saying there aren't any bad Gryffindors?'

Hagrid looked deeply uncomfortable, but eventually honesty won out. 'All right. It was a Slytherin that killed yer mum an' dad, but he couldn' have done it without help from a Gryffindor. One o' yer dad's closes' friends, at that.'

'So, not all bad people are Slytherins,' said Harry. 'And do you think all Slytherins except me are bad people?'

'No,' admitted Hagrid. 'No, they're not.' But he still sounded uncertain.

'Well, you can take as long as it takes to make your mind up,' said Harry. 'If my own aunt and uncle can hate me for being an orphan they were forced to foster and turning out to be a wizard and I can just ignore them, then if you want to hate me for being a Slytherin, I can ignore you, too. And if you want to be friends with my two best friends but not me, that's up to them to decide. But when Hiccup and Twigleg and Toothless are better and you want to invite people round for tea, you can decide then whether you hate me or not.'

And he strode off. He could hear Hagrid calling, 'Wait! I'm sorry! I didn' mean…' but he refused to look back.