Kevin had been looking forward to Moody's Defense Against the Dark Arts classes.
Barty Crouch Jr. had, under all the horror of what he was, been a genuinely effective teacher. He had taught the Unforgivables with real weight — made the students understand what they were, not as academic exercises but as things that had been used on actual people in actual rooms. His dueling demonstrations had been precise and brutal in a way that was pedagogically useful.
The real Alastor Moody, Kevin had hoped, would be all of that without the ulterior motive.
He had not counted on being assigned four years of Potions at the same time.
He rubbed the knot on his head from where Moody had found an opportunity to apply one during his brief visit to the staffroom, and accepted that this was his life now.
The first Potions class with the first-years had gone well enough that several of them appeared to have told other first-years about it, because when Kevin arrived at the classroom for his second session there were three students from a different house who had asked permission to observe.
He let them in.
The lesson was practical — foundational theory, then hands-on brewing, then a walkthrough of what had gone wrong for each student and why, specifically, without the kind of vague discouragement that made students afraid to ask questions. He was detailed without being condescending. He called errors by their correct names and showed their correction.
The students left with more information than they'd expected and somewhat more confidence than they'd had going in. The observers left with permission to come back.
Kevin returned to his workshop, put the kettle on, and found Dumbledore in his chair again.
"You're going to need a key," Kevin said.
"I have one, in a sense," Dumbledore said pleasantly. He was looking at the black velvet curtain Kevin had hung over a section of wall to cover a diagram he was working on. "May I?"
Kevin waved permission. Dumbledore drew the curtain back and studied the diagram for a moment — the schematic of the alert network, interconnecting circles, lines of dependency, notation in Kevin's hand running along the margins.
"This is more developed than I expected," Dumbledore said.
"I've been thinking about it since the summer."
"The synchronization problem you described." Dumbledore traced one of the lines with a finger, not touching the diagram itself. "Have you considered a blood-bond anchor?"
Kevin stilled. He had been approaching the problem from the signal side, working out how to encode a bearer's identity into the badge and transmit it reliably. A blood bond would solve it from the identity side — tie the badge to its bearer at a biological level so the distinction between this person is in genuine distress and this person is sparring became a function of their physiological state rather than a magical filter that could be spoofed.
"The degradation over time," Kevin said. "Blood bonds tend to weaken with distance."
"Only if the anchor is passive. An active bond — renewed periodically by the bearer — maintains its precision indefinitely." Dumbledore let the curtain fall back. "Nicolas and I worked through something similar in the 1930s. Different application, same underlying principle."
Kevin was already pulling out parchment.
He had, as of this conversation, three reasons to hope that Nicolas Flamel would eventually be persuaded to spend an afternoon at Hogwarts. He noted it on a mental list that was beginning to resemble an itemised argument.
Saturday mornings became a different kind of rhythm.
Dumbledore arrived at Kevin's workshop at eight and stayed until noon. They worked through alchemy at the pace of two people who were both considerably better at it than most wizards alive and were, therefore, working at a level where progress was measured in small precise increments rather than dramatic breakthroughs.
Hermione joined them after the second session, having absorbed Kevin's notes from the first two and arrived with questions that Dumbledore answered with the visible pleasure of a man who hadn't been challenged on fundamentals in some time.
Kevin watched Dumbledore teaching Hermione and reflected, not for the first time, that the Headmaster would have made a remarkable professor if he hadn't made a rather more remarkable everything else.
He filed that away too.
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