On Saturday afternoon, it was time for the "private coaching session" that William had arranged with the several class representatives.
Inside the office, the hour hand of the antique clock on the wall had just pointed to three, and before the bird even popped out to chime the hour, there was a knock-knock-knock at the door.
Percy Weasley pushed the door open impatiently, clutching a stack of neatly organized parchment in his arms.
He walked briskly to William's desk and set down the thick pile of homework, his face wearing the pride of someone who had completed a major mission.
"Professor, I've finished grading all the essays on 'The Characteristics and Defense of Dementors' that you assigned last class," Percy reported, puffing out his chest. "As per your request, I focused on checking their understanding of the theory behind the Patronus Charm."
William randomly pulled out a few sheets from the middle, quickly scanned the annotations, and nodded with satisfaction. "Very carefully graded. Thank you for your hard work, Mr. Weasley."
Having received praise, Percy looked delighted. Full of anticipation, he handed the parchment placed on the very top—his own assignment—to William.
William took the essay, glanced over it, and picked up a quill to write a large "E" (Exceeds Expectations) on it.
"Well written. Clear logic, proper citations. I can give it an 'Exceeds Expectations'."
The smile on Percy's face froze for a moment. He was somewhat surprised, and even said with a hint of defiance, "Professor, I thought I could get an 'Outstanding'. For this essay, I consulted all relevant materials outside the Restricted Section of the library."
"Diligence is commendable, but accuracy is more important."
William put down the quill and pointed to the second paragraph of the essay. "Look here. You wrote: 'When Dementors breed on a large scale in dark and damp environments, they produce mist.' However, it must be clarified that the word 'breed' cannot be used for Dementors."
Seeing Percy's confused expression, William explained patiently, "Dementors are different from normal magical creatures. They have no gender and do not possess biological reproductive capabilities. They are more like fungi growing in decay; they 'grow' out of thin air in places of despair, rot, and filth."
"This is a critical conceptual error. If this were in the O.W.L.s, the examiners would deduct points for this, and those points might just be what determines whether your grade is an 'O' or an 'E'."
Percy's face flushed. He hurriedly took out a quill and quickly corrected his essay under William's guidance, muttering about needing to write it down.
But that wasn't the end of it.
As a standard "overachiever," since Percy was here, he wasn't going to let this opportunity slip by easily.
He pulled out his textbook, The Dark Forces: A Guide to Self-Protection, which had colorful index tabs sticking out of nearly every page, along with a notebook filled with dense writing. He opened it to a dog-eared page.
"Professor, regarding the counter-curse mentioned on page 124, I still have a few questions..."
Just as Percy was asking away, the other class representatives—Charlie, Truman, Cedric, and Cho Chang—arrived one after another.
Seeing that Percy was in the middle of asking questions enthusiastically, they didn't interrupt but simply found seats quietly and waited.
Seeing that Percy seemed to have a hundred thousand whys, William checked the time. He hurried to answer a few of Percy's more profound questions, then had to cut him off.
"Alright, Mr. Weasley, we can leave the remaining questions for next time." William turned his gaze to the second-year student standing to the side. "Next, it's Mr. Diggory's turn."
Seeing Percy reluctantly packing up to leave, William stopped him. "Since you're here, why not stay a bit longer? You might gain something from other people's questions."
Although Percy was somewhat dismissive inwardly—thinking he already knew the basic knowledge of the lower years by heart and listening to their questions was a waste of time—on second thought, Charlie and the other upperclassmen were there too. Perhaps they would have more advanced questions. Plus, to show his eagerness to learn in front of the professor, he sat back down.
After guiding Cedric through revisions of his slightly immature essay, Cedric somewhat bashfully raised his confusion.
"Professor, regarding the Smokescreen Spell we learned last class... I always feel the smoke I produce during practice is very thin, not enough to obscure the opponent's vision. I want to ask how to improve the effect of this spell?"
"An excellent question." William stood up and gestured for Cedric to walk to the open space. "Show me."
Cedric took a deep breath and waved his wand. "Fumos!"
A stream of greyish-white smoke spewed from the tip of his wand. Although it enveloped the space between the two of them, in William's view, this layer of smoke was at best equivalent to mist on a cloudy day.
Although Cedric's face became somewhat blurry, the trajectory of his wand movements and his body language were still faintly visible.
"It's passable, but in actual combat, if the opponent can still see your wand movements and hear your incantation, the practical effect will be greatly discounted."
William waved his wand, and a gust of wind blew past, instantly dispersing Cedric's smokescreen.
Immediately following that, William flicked his wrist lightly.
"Fumos."
Without any large movements, a thick, almost solid grey smoke erupted instantly. In the blink of an eye, it filled half the office.
Percy and the others standing outside the smoke were surprised to find that they couldn't see the two people inside the smoke at all, as if a grey wall had suddenly been erected in front of them.
"Mr. Diggory!"
William's voice came from within the smoke, but it sounded muffled and distant, as if separated by a thick quilt. "Do you remember the specific effect of the Smokescreen Spell?"
To Cedric, William was clearly just a few steps away, but the transmission of sound was greatly hindered within the smokescreen, appearing indistinct and making it impossible to even discern the direction.
He was a bit nervous but answered loudly, "A spell used to create a defensive smokescreen, obstructing the enemy's vision and... and perception!"
"Very good. Now, cast a Disarming Charm with full force in the direction my voice is coming from!"
"But Professor, I can't see..."
"Do it!"
Cedric gritted his teeth and shouted forward, "Expelliarmus!"
A dazzling red light shot from the tip of his wand, rushing into the thick smoke and dust.
Although Cedric himself couldn't see clearly, the other class representatives watching from the side saw it perfectly.
When that originally fierce red light passed through the thick smokescreen created by William, it was like a stone falling into a quagmire.
It rapidly dimmed and distorted, its speed slowing visibly to the naked eye. Finally, just moments before it would have penetrated the smoke to hit William, it was completely swallowed by the smoke and dust, vanishing without a trace.
"This..."
Charlie and Truman's eyes widened, and Percy and Cho Chang's eyes shone with strange brilliance.
Because when they practiced this spell before, its use was mainly to obscure vision and harass opponents to facilitate escape.
They completely hadn't expected that this spell, regarded as "auxiliary interference," could be like the Shield Charm, so directly obstructing the opponent's spell through a physical medium.
Whoosh—
At this moment, William waved his wand.
The sky-filling smokescreen receded quickly like a tide, revealing an unscathed William and a bewildered Cedric.
"What are your thoughts?" William looked at the onlookers.
"That was amazing!" Charlie exclaimed. "I didn't expect the Smokescreen Spell could be used like that!"
"Completely unexpected..." Cho Chang also whispered. "That red light just disappeared directly."
From their reactions, Cedric roughly guessed the result just now and looked at his wand in shock.
William walked back to the desk and sat down again, looking at this group of Hogwarts elites with a profound gaze.
"The same spell, when used by different people, can produce vastly different effects. This doesn't seem to be a rare occurrence in the wizarding world; people usually attribute it to 'magical power'."
William paused and threw out a question. "But the textbooks don't explain in detail: aside from magical power, what exactly are the factors—even the minute ones—that cause this qualitative gap? Why can my smoke block spells, while yours can only obscure vision?"
No one answered. Percy frowned as he flipped through his book, Charlie mimicked the hand gesture from earlier, and Cho Chang bit her lip, seeming to ponder the composition of spells.
Seeing no one speak, all with heads bowed in thought, William's gaze circled the group.
He was going to pick someone.