The stool wobbled, the girl on top of it wobbled with it, and Leo crossed the last few feet of the aisle in two quick strides and put a steadying hand against the shelf.

"Whoa - careful, here, let me - which one did you want?"

The girl froze mid-stretch, fingertips still hovering an inch below the book she'd been reaching for, and looked down at him with an expression of such total startlement that he half expected her to topple off the stool from sheer surprise. She was small - smaller, standing on a stool, than she'd looked from across the room - swimming in an oversized grey cardigan with sleeves that fell past her knuckles, dark hair coming loose from a knot that had clearly been tied without the aid of a mirror. Behind a pair of slightly smudged glasses, her eyes were wide and quick and currently fixed on him with the frozen alarm of a creature caught in the open.

"The - " she started, and stopped. Swallowed. Started again. "The blue one. Third from the - the spine is blue. Quantum field. Volume two." The words came out in stiff little packets, clipped and precise, with a faint catch at the front of the harder ones. "I can reach it. I almost had it. The stool is sufficient."

The stool was, very visibly, not sufficient. Leo glanced up at the blue spine - a good handspan above where her fingers had been straining - and then at the alarming geometry of the girl, the stool, and the considerable drop to the hard floor, and made an executive decision.

"I believe you," he said diplomatically, "but how about you come down off the wobbly thing first, and I'll grab it. I'm taller. It's the only thing I've got going for me, honestly, let me use it." He offered a hand, not to pull her - just there, available, in case she wanted it for balance climbing down. "Easy does it."

She looked at his offered hand the way she might have looked at an unfamiliar piece of equipment, with wary analytical interest, and pointedly did not take it, climbing down from the stool on her own with a small graceless hop and immediately taking a step back to a safe distance, putting the stool between them like a tiny barricade.

Interesting, Leo thought. Not the usual reaction.

Because it wasn't. He'd grown wearily accustomed by now to the standard responses - the pink cheeks, the breathless fluster, the dazzled leaning-in. This girl did none of that. If anything she seemed to want him several feet further away than he was, and the look she was giving him wasn't adoration at all but something more like the deep suspicion of someone confronted with a variable that wasn't behaving according to specification. She'd retreated behind the stool and folded her cardigan-sleeves over her hands and was watching him with narrow scientific mistrust, and Leo found it, after a week of fizzing and squeaking, oddly restful.

He reached up - easily, no stool required - and drew the blue book down off the high shelf. Quantum Field Theory, Volume II. It was thick, dense, and aggressively unwelcoming, the kind of book that announced from its cover that it had no interest in being read by anyone who hadn't earned it.

"Here you go," he said, holding it out. "Volume two. Heavy. Good luck with it - this looks completely terrifying, by the way. Is this for a class?"

The girl took the book, again carefully avoiding any contact with his fingers, and clutched it to her chest like a shield. "It's not for a class," she said, and there was a flicker of something at the edge of the suspicion now - a faint affront, as though the suggestion that she'd read it for a class was mildly insulting. "The coursework here is - it's not - " she seemed to discard several uncharitable words " - it's not at the relevant level. This is for my own work. The volume one treatment of gauge symmetry was inadequate and I need the extended formalism."

Leo had understood approximately one word in three of that sentence, and the one he was most sure of was inadequate.

"Your own work," he repeated, genuinely impressed rather than just politely so. "So you're reading terrifying physics for fun. Because the school stuff is too easy." He didn't say it mockingly - there wasn't a trace of mockery in him - he said it with the frank admiration of someone encountering a skill entirely outside his own reach. "That's amazing. I can barely do the algebra they're giving me. What's the work? The thing the volume one was inadequate for?"

And something shifted.

Leo watched it happen, with the attentiveness he couldn't switch off - watched the wary suspicious creature behind the stool encounter a question she actually wanted to answer, and felt the change in the air the way you feel a room warm by a single degree. Because he'd asked. Not backed away from the terrifying book, not glazed over, not done the thing people presumably always did when she started talking about gauge symmetry - which was to nod and smile and edge toward the exit. He'd asked what the work was, like he wanted to know.

"It's - " She hesitated, clearly braced for his interest to evaporate the moment she actually started. Then, when it didn't, the words began to come, and they came faster and looser than before, the stiff little packets dissolving into something fluent and bright. "I'm modeling correlation decay in entangled systems. Most of the standard treatments assume idealized isolation, but real systems leak - they interact with the environment, decoherence, it's messy - and the interesting part, the actually interesting part, is the messiness, how the information dissipates, where it goes - " She was gesturing now, the book forgotten against her chest, her whole face transformed, lit from inside, the suspicion entirely burned away by the heat of the thing she loved. " - because it doesn't just vanish, that's the thing everyone gets wrong, it's not destroyed, it's distributed, it spreads out into the environment until it's just - unrecoverable in practice but not in principle - "

She stopped abruptly, as though she'd heard herself, and the light guttered, and the wariness rushed back in along with a flood of color to her face.

"- it's complicated," she finished, stiffly, retreating back behind the book. "You wouldn't - it's fine. You don't have to pretend."

"I'm not pretending," Leo said, and he wasn't. "I didn't follow most of that, I'll be honest. But I followed enough. The information doesn't get destroyed, it just spreads out so far you can't get it back." He tilted his head, turning it over. "That's kind of sad, actually. And kind of beautiful. Things don't disappear, they just scatter where you can't reach them anymore."

The girl stared at him.

It was a different stare than before. The suspicion was gone, and what had replaced it was a kind of arrested astonishment, as though he'd done something genuinely impossible - which, in a sense, from where she stood, he had. He had listened. He had listened to her talk about the thing nobody ever let her talk about, and he had not glazed over or fled or smiled the polite smile that meant please stop, and then he had taken the bright incomprehensible thing she loved and handed a small piece of it back to her, reshaped, a little sad and a little beautiful, in a way that proved he'd actually been listening with something more than his ears.

"That's - " she said, and her voice did the catch again, harder this time. "That's a - reductive but not inaccurate characterization." Which was, Leo would later learn, very high praise indeed. Her ears had gone pink. "Most people don't. Listen. To that part. They ask what it's for and when I tell them they - their eyes do a thing." She made a small gesture, a glazing motion with one cardigan-sleeved hand. "Yours didn't do the thing."

"Why would my eyes do a thing? You were talking about scattered information being unrecoverable but not destroyed. That's interesting. That's - I mean, that's a lot more interesting than my algebra homework." Leo smiled, easy and warm. "You really love it, huh. The physics. You light right up."

It was a perfectly ordinary observation. He meant nothing by it beyond what it said - that it was nice to watch someone care about something that much. He could not have anticipated, having no working model of his own effect on people, the result.

The girl's entire system, by all appearances, crashed.

The pink in her ears flooded down into her cheeks and up to her hairline. Her mouth opened and produced no sound. The fluent bright physicist of thirty seconds ago vanished entirely, replaced by someone who appeared to have suffered a complete and catastrophic loss of processing capacity, standing rigid behind a quantum field theory textbook with her eyes very wide and her face going an alarming shade of red.

"That's - " she managed, and the words came out scrambled, the catch turning into a full stutter. "That's not - the - the lighting in here is consistent, there's no - I don't - that's not a measurable - " She stopped. Visibly rebooted. "I have to recalibrate," she announced, to the bookshelf, with the air of someone reading an error message off the inside of her own skull. "There is an - anomaly. In the - I'm experiencing an anomaly. It's not - it's probably the - "

"Are you okay?" Leo asked, concerned now, because she'd gone from incandescent to alarmingly flushed in the space of one sentence and was no longer entirely making sense. "Do you need to sit down? Is it warm in here? I can open a window -"

"Don't open the window!" The vehemence of it surprised them both. She clutched the book tighter. "The temperature is - the window is not the variable. I'm fine. I'm completely fine. I have to go now. To recalibrate. Goodbye."

And she fled - clutching her terrifying physics book to her chest, cardigan sleeves flapping, around the end of the stacks and gone, leaving Leo standing alone in the quiet aisle, the abandoned stool the only evidence she'd been there at all.

"...Okay," Leo said, to the empty library, for what felt like the hundredth time since he'd arrived in this world. He looked at the wobbly stool. He looked at the gap on the high shelf where Quantum Field Theory, Volume II used to live. "Bye, then. Glad I could - help. With the book."

He genuinely could not work out what he'd done this time. He ran back through it: she'd wanted a book, he'd gotten the book, they'd had a nice conversation about scattered information, he'd said she lit up when she talked about physics - which was just true, plainly, observably true - and somewhere in there she'd short-circuited and bolted like the library was on fire. It was baffling. She'd been the most normal person he'd talked to all week, right up until the exact second she abruptly wasn't.

He sighed, righted the wobbly stool, and went to find his own quiet corner.

He'd come to the library for refuge from being noticed, and instead, without the faintest idea he'd done it, he had once again - simply by listening, simply by being kind, simply by treating a strange brilliant lonely girl like a person worth listening to - achieved the precise opposite of going unnoticed.

And somewhere in the building, a girl in an oversized cardigan was hurrying through the corridors with a textbook clutched to her chest and a heart rate she could not account for by any environmental variable, repeating to herself with mounting scientific alarm that there had to be a rational explanation, there was always a rational explanation, she would identify the cause and isolate it and bring her own malfunctioning system back under control through the careful application of method -

- and trying, very hard, not to think about the way he'd said the information doesn't disappear, it just scatters where you can't reach it, as though he'd understood something about her that she had never managed to put into words herself.