Chloe Vance had a hypothesis, and she did not like it.

She sat at her desk at six in the morning - she had not slept well; she had not, if she was being rigorous, slept much at all - with a band of her own design strapped around her wrist and a second sensor clipped to her fingertip, both feeding into the small device that now lived on her desk like a confession she couldn't put down. The display glowed in the pre-dawn dark. A single number sat on it, steady and unremarkable.

Sixty-two beats per minute. Resting baseline. Calm. Normal. The number of a girl alone in her room at dawn with nothing to disturb her, which was precisely the controlled condition she'd wanted.

"Good," Chloe murmured to the dark. "Baseline established. Now. Methodology."

The methodology was sound. It was, in fact, the only part of this entire humiliating enterprise she felt confident about. If her body was producing anomalous outputs, then the responsible thing - the scientific thing, the only thing that gave her any sense of control over a situation that had comprehensively escaped it - was to characterize those outputs precisely. To establish the baseline, introduce the suspected stimulus under controlled conditions, and measure the response. You did not panic about a phenomenon. You measured it. Measurement was how you turned a frightening unknown into a known quantity, and known quantities, however unwelcome, could not sneak up on you.

The suspected stimulus was a boy named Leo Evans.

The controlled introduction of the stimulus, given that it was six in the morning and he was not present, would have to be conducted via recall. She would summon him to mind - deliberately, under observation - and measure what her own traitorous cardiovascular system did about it.

"Trial one," Chloe said, and noted the baseline on the pad beside her. Sixty-two. "Introducing stimulus."

She thought about Leo.

She didn't even have to try hard, which was itself a data point she chose not to dwell on. She simply let the image come - the easy way he'd held out the umbrella, handle first, you need it more - and the number on the display moved.

Sixty-two. Sixty-eight. Seventy-four. Seventy-nine.

Chloe stared at it.

"...Recording elevated reading," she said, faintly. She wrote it down with a hand that was not entirely steady. "Seventy-nine. From baseline. On - on recall alone. That's a seventeen-beat increase from a memory." She cleared her throat, and made herself say the next part out loud, because saying it out loud was part of being rigorous and she would not let herself off the hook. "That is not within normal parameters for - for thinking about a person one has merely met."

She ran it again. She was nothing if not thorough, and one trial was not a result.

Trial two: she recalled the library. The way he'd said you're really cute, you know that, fond and unguarded, looking right at the part of her nobody looked at. The number climbed to eighty-six and the device, helpfully, emitted the soft warning chirp she'd programmed it to make at the eightieth percentile, as if to underline the point.

Trial three: she recalled him jogging off into the rain, soaked and laughing, waving without turning around. Eighty-one.

Trial four, the control - she deliberately thought about something neutral, the gauge symmetry problem she'd been stuck on, to confirm the effect was specific to the stimulus and not just general arousal of the nervous system from the experiment itself. The number sank obediently back toward baseline. Sixty-five. Calm. The physics did not make her heart race. The physics had never made her heart race; the physics was where she went to make everything stop racing.

Trial five: back to the stimulus. Back up to the eighties.

Chloe set down her pen.

The data was unambiguous. It was, in fact, some of the cleanest data she had ever collected - a clear, repeatable, dose-dependent response, specific to a single stimulus, robust across trials, returning to baseline under control conditions and spiking reliably on reintroduction. If she'd been presenting it in any other context she'd have been quietly proud of the experimental design. As it was, she sat in the grey dawn light and looked at the neat damning columns of numbers and felt the floor of her carefully ordered world tilt.

Because she'd known, going in, what she was hoping for. She'd been honest enough with herself to admit the hypothesis she wanted to be true: that this was something else. A virus, maybe - some low-grade infection elevating her resting rate. An anxiety response, generalized, that merely happened to surface around a salient new person. A caffeine sensitivity. A cardiac irregularity, even - she'd have taken a cardiac irregularity, gladly, a nice clean medical problem with a name and a treatment and nothing whatsoever to do with the unbearable possibility she was now staring at in glowing red.

She ran through them, the alternatives, one by one, the way you check off a differential diagnosis, and crossed each one out.

Not a virus: no fever, no other symptoms, and viruses didn't care which specific person you were thinking about.

Not generalized anxiety: the control trials were clean. Neutral thoughts, neutral readings. The response was specific.

Not caffeine: she'd had none.

Not a cardiac problem: the baseline was textbook, the elevations were stimulus-locked, the recovery was immediate and complete. There was nothing wrong with her heart. Her heart was, mechanically, in perfect working order.

It was just doing this. About him. Only about him. Reliably, repeatably, undeniably about him.

"...Oh," Chloe said, very quietly, to the empty room and the glowing number and the borrowed umbrella still standing in the corner where she could see it. "Oh, no."

She had, in the language she trusted, exhausted the alternative hypotheses, and the one that remained - the one she'd been running this entire desperate dawn experiment specifically to rule out - was the one the data supported.

She liked him.

She made herself think the word, the actual word, the unscientific embarrassing human word, because dressing it up in physiology was just another way of not looking at it and she was done not looking at it. Not elevated cardiovascular response to a salient stimulus. Liked. She liked Leo Evans. The seventeen-beat jump was what liking someone did to a person, apparently; it was, she understood now with the particular vertigo of someone for whom a universal human experience has arrived embarrassingly late and entirely without warning, simply what this felt like from the inside. Everyone else presumably learned this at thirteen. She was learning it at sixteen, alone, at dawn, with instrumentation, because she had never once in her life had cause to learn it before.

And here was the part that frightened her - not the liking itself, though that was alarming enough, but what came after the liking, the great blank terrifying region her data couldn't map at all.

Because now what?

That was the question that made the elegant little experiment useless. She had characterized the phenomenon beautifully. She had quantified it, isolated it, confirmed it. And it had bought her precisely nothing, because the phenomenon was not a physics problem, and Chloe had spent her entire life retreating from exactly this - from the messy unquantifiable domain of other people, of feelings, of the things that had no formalism and obeyed no laws and could not be solved with sufficient cleverness. People didn't have closed-form solutions. You couldn't derive the right thing to say to a boy from first principles. There was no extended formalism for what to do when your heart rate spiked seventeen beats over a memory of someone being kind to you in the rain.

She was good - she was the best, she was a genius, everyone said so - at exactly one kind of thing, and this was not that kind of thing. This was the other kind. The human kind. The kind she'd always failed at, the kind that had made her the awkward one, the strange one, the sister who couldn't manage a conversation while Scarlett managed entire committees, the girl who'd learned to stop trying to be a person and just be a mind instead because the mind, at least, was something she was allowed to be good at.

And the worst of it, the thing that put the hot prickle of tears behind her eyes there in the cold dawn, was the oldest fear of all, surfacing now with new and specific teeth: that even if - if - she could somehow learn the unlearnable thing, somehow cross the blank region, somehow find the words and the way - he would only ever like the mind. That was what people liked, when they liked anything about her. The processing core. The useful part. And Leo had said cute, had looked at the human part and called it cute, and that was the most precious and most terrifying thing anyone had ever done, because now she had something to lose - now there was a version of this where he saw the cute flustered human girl and not just the genius, and wanted her, the actual her - and the existence of that possibility meant the existence of its opposite, the version where she reached for it and found out it had been the mind all along, the way it was always the mind, and that would be a kind of confirmation she did not think she could survive.

Chloe pulled off the wrist sensor. The number on the display, freed of its stimulus, settled slowly back toward baseline - sixty-four, sixty-three, sixty-two. Calm. Resting. As though nothing at all had happened, as though the girl wearing it hadn't just, with her own instruments, in her own handwriting, proven the most destabilizing thing she'd ever proven.

She looked at the data one more time. Then, instead of deleting it, she saved it - labeled the file, dated it, filed it carefully, because it was hers and it was true and she did not throw away true things - and she sat for a while in the strengthening light with her chin on her knees and the umbrella in the corner of her eye, and let herself, just for a few minutes, simply feel the enormous frightening size of it without trying to measure anything at all.

Then, because she was who she was, she reached for a fresh page and began, despite everything, to plan.

If the problem was that she didn't understand the variable - didn't understand him, what he was, why he did the irrational things he did, whether the way he'd looked at her meant what some hopeful unscientific part of her wanted it to mean - then the answer was the same as it always was. More data. She needed more data about Leo Evans specifically, not just her own response to him. She needed to understand the phenomenon from the other side. And there were, she thought, ways to do that. Systematic ways. Ways involving, perhaps, instrumentation of a more ambitious sort than a wrist sensor - something that could assess not just her own readings but the interaction itself, the compatibility of the system, the -

The idea began to glow, in the dangerous way her best and worst ideas always did.

She wrote, at the top of the fresh page, in her small ferocious hand: PROJECT - COMPATIBILITY ASSESSMENT. Then she underlined it once, and looked at it, and felt the specific fragile dangerous flicker of something she had not let herself feel in a very long time and could not quite call by its name.

It looked a great deal like hope.

Outside, the sun came up over Oakhaven. Down the hall, her sister's alarm went off, and Chloe heard Scarlett rise and begin the brisk efficient machinery of her morning, the same as every morning. Two sisters in one house, getting ready for the same school, neither knowing that the other had spent the night before in a quiet room keeping the strangest and most secret kind of records about the same boy - and that today, without intending to, without any idea it was coming, they were going to find out.