The old man came on the second day.

  Chen Xi — he was still thinking of himself as Chen Xi, because the alternative was thinking of himself as a dead man in a stolen body, and that particular line of reasoning was not productive — had spent the night sitting upright against a breastplate that was large enough to have belonged to something considerably bigger than a human. He had not slept. The pain in his chest made sleep theoretical. Instead, he had counted bone frequencies until he had a dataset of 147 measurements, which he stored in memory using a mnemonic system he had developed in graduate school for remembering experimental parameters.

  He was arranging bones by frequency when the old man appeared at the edge of his peripheral vision and stopped.

  The old man was small and weathered in the specific way that suggested not age but erosion — as though he had been exposed to something abrasive for a very long time and the softer parts of him had been worn away, leaving only the dense, essential core. His clothes were made from material Chen Xi could not identify: too rough for cotton, too flexible for canvas, stained the same brown as the earth. His eyes were sharp and cautious in the way of an animal that has survived by being the first to notice movement.

  They looked at each other for a while.

  "You're not dead," the old man said, in a language Chen Xi should not have understood but did — the words arriving in his mind pre-translated, as though the body he was wearing came with a built-in phrasebook. This was, he noted, an extraordinary phenomenon that he would investigate thoroughly as soon as he had addressed more immediate concerns.

  "No," Chen Xi said.

  "You should be. You've been lying in the Silted Bones for two days with a shattered dantian and no cultivation to speak of. The residual Qi alone should have dissolved your meridians."

  Chen Xi absorbed this sentence the way he absorbed experimental data that contradicted his models: carefully, without prejudice, filing each unfamiliar term for later investigation. Dantian. Cultivation. Qi. Meridians. Four unknowns. He needed definitions.

  "What is a dantian?" he asked.

  The old man stared at him. It was the stare of someone reassessing a situation they thought they understood. "You don't know what a dantian is."

  "I do not."

  "You're a cultivator — or you were. Your body has meridians. Your core is shattered. You have remnant Qi signatures. And you don't know what a dantian is."

  "I know what a sternum is," Chen Xi offered. "And a diaphragm. The thing you're describing is between them, approximately four centimetres below the xiphoid process. It feels like broken glass. Is that the dantian?"

  The old man sat down heavily, which seemed less like a choice than a surrender to gravity. "What's your name?"

  "Chen Xi." Then, after a pause in which he considered the implications of his situation more carefully: "Though I suspect the name on this body is different."

  "This body."

  "Yes. This is not my body. I died. I was a physicist — a scholar of natural forces. I was conducting an experiment, and there was an accident, and I died, and then I was here, in this body, which has a broken... dantian... and hair past my ears. I would like to understand what has happened, and I would like to understand what Qi is, because I have been measuring an energy phenomenon in these bones for approximately thirty-one hours and I believe the two are related."

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  The old man was quiet for a long time. The bruise-coloured sky shifted overhead. A wind moved across the Silted Bones, carrying dust that smelled of old copper.

  "My name is Wu Zheng," the old man said. "I have been in this graveyard for seventy-three years. I was an elder of the Azure Dust Sect before they shattered my cultivation and threw me here to rot. I have survived by eating spirit moss and the flesh of minor beasts and by being too stubborn to die." He paused. "You are either the most convincing madman I have met in seven decades, or you are something I have no explanation for. Either way, I can show you what Qi is. Give me your hand."

  Chen Xi gave him his hand.

  Wu Zheng closed his eyes. The air between them changed — not visibly, not audibly, but Chen Xi felt it the way he felt static charge before a thunderstorm: a shift in some local field that his body was apparently equipped to sense. A warmth entered his palm. Not thermal warmth. Something else. A pressure that had direction and rhythm, that moved through the skin of his hand and into the channels — meridians, he supposed — beneath.

  He watched the energy move. Not with his eyes. With whatever sense organ this body possessed that his original had not. The warmth had colour, somehow — a pale blue-white, like Cherenkov radiation, like the glow of charged particles moving through a medium faster than light could follow. It entered his hand, moved up his forearm, reached the first junction point in his elbow, and stopped. Not gradually, the way water slows in a widening channel. Abruptly. As though it had hit a wall.

  "Your meridians are damaged," Wu Zheng said. "The energy can't circulate properly."

  Chen Xi was not listening to Wu Zheng. He was counting. The energy had entered his palm at a measurable rate. It had traveled through his forearm at a measurable speed. It had encountered an obstruction at a specific, locatable point.

  "Do that again," he said.

  "What?"

  "The energy. Send it again. Same amount, if you can."

  Wu Zheng frowned but complied. Again the warmth entered. Again it moved through the forearm. Again it stopped at the elbow junction. Chen Xi measured the transit time. Compared it to his first measurement. Consistent. The energy moved at a fixed speed through this medium.

  "Again," he said.

  "Why?"

  "I need a third data point to confirm the rate is constant."

  "The rate of what?"

  "The energy's propagation speed through biological tissue. If it's constant, it obeys a transport equation. If it obeys a transport equation, I can model it. If I can model it, I can optimise it."

  He said this with the flat certainty of a man stating that water flows downhill. Wu Zheng sent the energy a third time. The rate was constant.

  Chen Xi smiled. It was the first time he had smiled since dying, and it was not a warm smile, or a relieved smile, or any of the smiles that a normal person might produce upon receiving good news. It was the smile of a man who has just discovered that the foreign country he woke up in runs on the same mathematics as home.

  "This energy," he said. "This Qi. It conserves."

  "It what?"

  "It can't be created or destroyed. Only moved, transformed, concentrated, dispersed. The total amount in a closed system remains constant. Your body takes it in, processes it, loses most of it as waste, and retains a fraction. The retained fraction builds up over time. That's what cultivation is, isn't it? Accumulation."

  Wu Zheng opened his mouth. Closed it. Opened it again.

  "Yes," he said slowly. "That is... a very crude description of cultivation."

  "How efficient is the retention?"

  "What?"

  "What fraction of the energy you take in do you actually keep? One in ten? One in twenty?"

  "I don't... we don't measure it like that."

  "Estimate."

  Wu Zheng considered. He had been a cultivation theorist before his fall — one of the Azure Dust Sect's better ones, which was why his ideological deviation had been considered dangerous rather than ignorable. He had spent decades studying energy circulation and had never once framed the question in terms of efficiency ratios.

  "Perhaps... one part in twenty?" he said. "For a typical technique at my former level. The body absorbs spiritual energy from the environment, circulates it through the meridians, refines it in the dantian, and stores what it can. Most is lost during circulation. The meridians are... imperfect conductors."

  "Five percent," Chen Xi said. He said it the way a structural engineer might say "the bridge is made of cardboard." Not angry. Horrified. "You retain five percent of the input energy."

  "That is considered adequate."

  "Adequate." Chen Xi pressed his palm against the broken thing in his chest. Five percent. A ninety-five percent energy loss in a biological transport system. That wasn't a cultivation method. That was a haemorrhage with extra steps.

  He sat in the dust among the ancient dead and began to think about fluid dynamics.