If...

Severus had worked meticulously on his plan, attending to every detail as an alchemist would to his most precious formula. He walked slowly through his dungeon classroom, his footsteps muffled in the silence, his piercing gaze sweeping over every workbench, every cauldron, every vial. Here, the scent of sulphur and rare herbs mingled with the flickering shadows of torches, creating a heavy, almost solemn atmosphere. It was into this ambience that Severus hoped to weave the web of his deception.

For a seemingly ordinary lesson, he had chosen Swelling Solution, a potion usually reserved for second years. Simple to him, a mere formality, but an insurmountable challenge to the first years - especially the Hufflepuffs, who Severus knew were less adept at the technical demands of delicate potions. For these young, inexperienced wizards, mistakes were inevitable: exploding cauldrons, potions of strange colours, ominous fumes - the harbingers of disaster.

But Severus had a far greater goal in mind. This staged scene served a larger purpose: to extract vital information about Voldemort from Slughorn. The old Potions Master knew things, secrets hidden behind his warm smile and his love of the finer things in life. Severus was no fool, and he intended to exploit the professor's weaknesses, even if it meant swallowing his own pride - if only for a moment.

He had no intention of making a fool of himself or appearing incompetent. No, he would maintain an air of seriousness, a look of rigour, that of a slightly over-ambitious teacher with excessive expectations. That would be enough to attract Slughorn's attention, who wouldn't resist leaning in and offering friendly advice. He could already picture his former professor, eyes gleaming with feigned humility, offering a few 'tips' on how to 'lighten the students' load', as if taking Severus under his wing to help him temper himself. Severus would merely nod, appearing thoughtful, all the while preparing every word of their future conversation with the precision of a duellist, ready to bypass Slughorn's defences and extract every vital detail.

As the students began to file into the room, Severus straightened and fixed his gaze on the door, his expression unreadable. The first to cross the threshold were the Ravenclaws, neat and quiet, followed closely by a slightly more boisterous group of Hufflepuffs. Severus waited until everyone had taken their seats, discreetly observing their youthful faces and palpable nervousness. He knew that, faced with this swelling solution, their inexperience would betray them, and that was exactly what he hoped would happen.

As soon as the last student was seated, he began the lesson, his voice slow and deep.

"Today," he announced, deliberately stressing each syllable, "you will prepare a Swelling Solution. This potion requires a level of mastery that, let's be honest, most of you will not achieve by the end of the year, or perhaps ever."

As expected, one Ravenclaw student dared to raise his hand. Severus already knew what he was going to say.

"Sir? I don't think the Swelling Solution is on this year's syllabus."

"What did I say about relying on the textbook? I make the syllabus," Severus replied, his tone cutting.

The student lowered his eyes and fell silent. Severus' eyes swept slowly down the rows. Several students exchanged anxious glances, while the more confident ones picked up their quills, ready to write down any instructions. Severus allowed himself a barely perceptible grin. The fear on their faces gave him a sense of power. Lily would surely scold him, but the end justified the means for the greater good.

"Follow my instructions to the letter," he continued, nodding at the blackboard. "Any inaccuracy, any lapse in concentration... and your cauldrons will become bombs waiting to explode. This is not an exercise for idiots."

As they began, Severus moved between the rows with calculated slowness, watching their movements, noting their mistakes without intervening. A few Hufflepuffs had already got off to a bad start, adding too much or too little nettle to their mortars, crushing the fisheyes poorly... It was child's play, really. If he hadn't been expecting disaster, Severus would have given these inattentive students a thorough scolding.

The minutes ticked by, and a faint greenish smoke rose from one of the cauldrons, irritating the noses of those nearby. Severus held back an exasperated sigh, struggling to maintain a neutral, almost bored expression, as if he were witnessing an expected, almost entertaining spectacle. Slughorn would soon be passing through the corridor, and soon one of those damned cauldrons would overflow, or better still... explode!

Severus didn't have to wait long. Barely a few minutes later, a sinister gurgling sound echoed from a corner of the room, followed by a muffled 'boom'. A thick cloud of olive-green smoke rose from a Hufflepuff's cauldron, accompanied by the sickening stench of rotten eggs. The disaster he had been waiting for had been served to him on a silver platter.

Severus was jubilant inwardly, but showed no sign of it as the students near the scene began to cough and fidget. Severus slowly raised an imperious hand, imposing silence without a word. He approached the offending cauldron, his face impassive, his dark gaze darting from one panicked student to another.

"Blatant incompetence, Mr Billbucket," the young teacher murmured coldly. "The instructions were amazingly simple."

Petrified, Billbucket dared not look Severus in the eye, preferring to stare at his disastrous cauldron instead. Severus narrowed his eyes and made a sharp motion with his wand to clear the smoke.

"You're in luck, Mr Billbucket. Your notorious ineptitude has spared you..."

As he scolded the student, a movement in the shadows caught his attention. As planned, Slughorn had appeared in the doorway, looking alarmed, his eyes searching the room for the source of the disturbance.

"Severus, my boy, are you all right?" asked Slughorn, his voice a mixture of concern and forbearance.

The second miracle had occurred, and Severus paid no attention to Slughorn's almost paternal attitude towards him. He did, however, straighten up and put on a falsely irritated expression, as if torn between frustration and humiliation.

"I'm afraid these students are suffering from acute idiocy, Horace."

Sensing an opportunity to play mentor, Slughorn entered the classroom uninvited, his gaze lingering briefly on the blackboard where Severus had written the instructions for the day's potion.

"Ambitious," the elder professor remarked, a smile on his lips. "Let me assist you, my dear Severus."

Slughorn approached the cauldron, still emitting green wisps, and in front of the whole class and Severus, he corrected the mistake with a precise flick of his wand. The strange concoction created by Billbucket vanished completely.

Then Slughorn checked each cauldron, rewarding the students with encouraging, sometimes flattering comments. Finally, he returned to Severus, who had returned to his desk, and said in a low voice:

"This isn't the protocol from the handbook, is it?"

"No... I'm experimenting."

"And this potion seems a bit premature, especially as it's on the second-year syllabus. My boy, you're making things difficult for yourself - and for your students. Come to my office after class. I'll pour you a nice cup of tea and we'll discuss it.

The whole exchange was accompanied by a genial smile that, under different circumstances, might have made Severus nauseous. Slughorn bade the students a hearty goodbye and left the classroom.

Less than an hour later, Severus knocked on the door of his colleague, who invited him to sit down in a very comfortable leather chair. Slughorn had always appreciated comfort and luxury. His office and home reflected the image of a socialite, a bon vivant.

The young man, watching his former professor pour him a cup of tea, was determined to gain his trust, even if it meant appearing as a novice teacher seeking advice.

"My dear Severus, I always knew you would become a meticulous, exacting potions master," he said, handing him a cup. "When Dumbledore suggested your name to ease my schedule, I agreed immediately. You were my best student, along with Miss Evans..."

A shadow of sadness crossed Slughorn's eyes.

"I should say, Mrs Potter... What a tragedy, really... I can't imagine what she's going through after losing her husband... and her poor little boy. I remember writing to her a few weeks before the tragedy, mentioning in my letter that you had been recruited to Hogwarts... I had so many students, but if I remember correctly... you were friends, weren't you? Oh, I remember! You were an inseparable pair until your sixth year when, for some reason, you drifted apart. It was rare, a Slytherin and a Gryffindor working together... I hope it wasn't a falling-out between you. Perhaps you could tell me, dear Severus, how Mrs Potter is holding up since this terrible loss. Dumbledore is silent on the subject, but since you were close, perhaps you know more. "

Severus hadn't anticipated that his plan might backfire. It was imperative that no one - especially Slughorn - found out that Lily was living under his roof. Nor could he allow any hint that they had rekindled their friendship. If the Slug Club meetings had taught him anything, it was that Slughorn corresponded regularly with Barnabas Cuffe, the editor of the Daily Prophet. The last thing he wanted was for one of his confidences to make it to the front page and jeopardise Lily and Harry's safety.

"Unfortunately, no, Horace. Mrs Potter and I ended our friendship at the end of our fifth year. I don't want to go into the reasons for our falling out."

"I'm sorry to hear that, Severus. Friendships come and go, alas! I've read some speculation in the Prophet... But I've written to dear Barnabas Cuffe, asking him to stop publishing such nonsense. He's always taken my opinion into account and occasionally consults me when he needs an idea for a report".

For Lily's sake, Severus was willing to listen to Slughorn boast about his extensive address book. But it was imperative that he regained control of the conversation.

"Let's move on, dear Severus. You didn't come here to listen to an old man reminisce. I wanted to talk to you about your lesson... Don't worry, my boy, I thought you were brilliant but a little too ambitious."

"You're probably right, Horace," he admitted with calculated modesty.

Severus took a sip of his tea, which had a bitter orange flavour.

"You have to know how to gauge the difficulty, especially with Hufflepuffs, who are so impressionable. Some of these young ones will never handle a vial properly without a bit of... guidance," Slughorn explained with a hearty laugh.

"I understand perfectly," Severus agreed, his face impassive.

"Ah, Severus, that's one of the great mysteries of teaching! You must know how to balance things... a bit like potions, you see? Sometimes it's pointless to insist when you know an ingredient is stubborn," he said with an amused wink. "I noticed it myself when you were struggling. Your students need guidance, especially the Hufflepuffs, especially young Mr Billbucket... A walking disaster if not watched closely."

Severus grinned sardonically... Poor Tommy Billbucket.

"But you must admit, my friend, that today's lesson was a little too ambitious for this class. I know, I know... The Swelling Solution may seem like child's play to those who have a knack for it, like you and me, and a few gifted students... But for the others, for those who don't share our natural talent in this area, it's better to stick to the syllabus... Hmm... By the way, dear Severus, why weren't your students using their textbooks today?"

"For practical and pedagogical reasons, I have decided to dispense with them, Horace. Everything my students need is on the board. If they need additional information, I encourage them to ask me".

"I wouldn't presume to criticise you on that point, my boy. I've seen too many students forget their books. But you should know that some students find it reassuring to have their textbooks close at hand. They can get the information they need without disturbing you when you're occupied elsewhere.

"I'll keep that in mind, Horace."

"And, between connoisseurs, I wanted to say that I was impressed by the protocol you gave your students. I wasn't convinced at first, but on reflection I must admit that the use of the dried nettle is quite relevant. Soon it won't be you who comes to me for advice, it will be me who comes to you for expertise, my boy. You can't imagine how gratifying it is to see one of your students become a true genius!

Severus' talent owed nothing to Slughorn, far from it. And yet the old professor seemed convinced that it was his teaching that had awakened Severus's gift for potions. Once again, the young man resigned himself to swallowing his pride to lure Slughorn into his web.

"I can never thank you enough for being such a good teacher to me, Horace."

"Oh, it's nothing, really!" replied the old teacher with a pleased smile. "Just know that I'm happy to help you, especially now that we're colleagues!"

A new smile appeared on Severus' lips, a victorious one. Horace Slughorn did indeed seem ready to welcome him, answer his questions and listen to him. But there was no need to rush things by revealing his intentions too quickly. True to his Slytherin nature, Severus knew that, like a snake, he had to watch his prey with patience, hypnotising it with words before he struck. Cunning was Severus' middle name.