The days in Liyue Harbor settled into a gentle, wondrous rhythm. Mornings were for exploring, afternoons for quiet time in the garden, and evenings were for family. Ren was growing more accustomed to the bustling energy of the city, yet it was during the quiet hours, when the sun went down and the city's myriad lanterns began to glow, that he noticed something fundamental.
It was cold.
Not the biting, unforgiving cold of a snowy mountain, but a persistent, creeping chill that seeped in from the sea. It settled in the corners of the beautiful house, making the polished wood floors feel like sheets of ice under his bare feet. He would burrow under the thick, silk-filled blankets Ganyu had provided, feeling cozy and warm, but the air outside his cocoon remained crisp and cool.
During one of his lessons with Xianyun, she mentioned the hardships of those living in the mountain villages of the Qiongji Estuary. "In winter, the chill descends from the peaks," she explained. "Many common folk rely on thick clothing and well-stoked hearths. Some of the more affluent households are able to acquire pyro crystals, which emanate a constant, gentle warmth."
The idea stuck with Ren. A pyro crystal. A magical, ever-warm rock. It was a fascinating concept, but his mind, a fusion of a curious child and a long-dormant engineer, immediately saw the flaws. Constant. Unregulated. What if it was too warm? What if you only needed heat at night? It seemed… inefficient.
The next day, when Ganyu arrived in the afternoon, Ren had a special request.
"Big sister Ganyu," he asked, looking up at her with his most earnest expression. "Can you get me some things? I need… drawing paper. A lot of it. And some fine charcoal sticks. The kind architects use."
Ganyu blinked, then her face broke into a doting smile. "Of course, Ren! Are you going to draw something for me? A picture of the garden, perhaps? Or maybe our house?" The thought of a cute, childish drawing from her little brother to hang in her office filled her with a warm, fuzzy feeling.
"Something like that," Ren said evasively, a spark of intense focus already lit behind his glowing azure eyes.
True to her word, Ganyu returned within the hour with a portfolio of high-quality rice paper and a box of expertly crafted charcoal sticks of varying thickness. Ren thanked her profusely, took the materials to the low table in his room, and closed the sliding screen door.
For the next two days, he was a whirlwind of quiet, focused activity. Ganyu would peek in on him, her heart melting at the sight. He would be sprawled on the floor, surrounded by sheets of paper, his tongue sticking out of the corner of his mouth in concentration as he drew. She saw lines, circles, and strange, boxy shapes. She assumed he was creating an elaborate, imaginary world, and she was content to let him work, leaving snacks and juice by his door with a soft knock.
On the evening of the second day, he emerged, looking tired but triumphant. He was holding a single, rolled-up sheet of paper.
"I finished it," he announced simply, holding the scroll out to Ganyu.
"Oh, Ren, let me see!" she said, her voice full of loving indulgence. She took the scroll and carefully unrolled it on the main table.
The indulgent smile on her face slowly faded, replaced by an expression of utter bewilderment.
This was not a child's drawing.
The paper was covered in precise, ruled lines and perfect curves. It was a technical diagram, meticulously detailed and annotated with neat, blocky Liyue script that Xianyun had been teaching him. It depicted a metal box with vents on the top. There were cutaway views showing an intricate internal assembly: a slot for what was labeled an 'electro crystal', a series of thin, branching lines labeled 'energy-transferring wires', and at the core, a tightly wound spiral of metal labeled 'heating coil'. There was even a separate, smaller diagram for a rotating dial on the side, connected to something he called a 'regulator'.
It was, without a doubt, a blueprint.
"Ren… what is this?" Ganyu whispered, tracing a finger over a line that depicted energy flowing from the crystal to the coil.
"It's a heater," Ren explained, puffing out his chest with a hint of pride. He then pointed to the diagram. "See? You put an electro crystal here. The energy flows through these wires to the coil. But the coil is made of a special metal that… that doesn't like the energy passing through it. It fights back." He was trying to explain the concept of electrical resistance in the simplest terms. "When it fights, it gets very, very hot. The heat comes out of the vents and warms the room."
He then pointed to the dial. "And this is the best part! It's a regulator. It's like a little gate. You can open it a tiny bit to let just a little energy through for a little heat, or you can open it all the way for a lot of heat. You can turn it on and off! It's not like a pyro crystal, which is just… on. All the time. This is better, and you don't waste the crystal's power when you're not cold."
Ganyu stared, her mind, which was used to processing complex logistical data, struggling to comprehend what she was seeing. The concepts were completely alien, yet the logic, as he explained it, was undeniable. This was not magic. It was a machine. A machine designed by a ten-year-old child.
"Master," she called out, her voice a little shaky. "Master, I think you need to see this."
Xianyun emerged from her room, her red glasses perched on her nose. She glanced at the blueprint, a flicker of mild curiosity in her eyes. "A new drawing, Ren?"
Ganyu simply handed her the sheet of paper. Xianyun took it, and as her golden eyes scanned the diagram, the air around her changed. Her placid, scholarly demeanor evaporated, replaced by an aura of intense, razor-sharp focus. Her eyes, magnified slightly by her glasses, narrowed. She traced the same lines Ganyu had, but her mind was seeing something entirely different. She wasn't just seeing a diagram; she was seeing the flow of energy, the principles of physics, the sheer, breathtaking elegance of the design.
Ren, feeling a little intimidated by her sudden silence, decided to explain it again. He spoke of heat dissipation, of how the coiled shape maximized the heating surface in a small space, of how the entire mechanism could be constructed by a skilled mortal blacksmith with no need for adeptal arts.
"The cold at night made me think," he finished, his voice small. "Some people in the mountains must be very cold. Pyro crystals are good, but this… this is feasible. And it can be regulated."
Xianyun remained silent for a full minute after he finished, her gaze locked on the blueprint. The only sound in the room was the gentle hum of a single lantern. Ganyu held her breath.
Finally, Xianyun looked up, not at the blueprint, but directly at Ren. She looked at him with an expression he had never seen before. The fondness was still there, the protectiveness was still there, but now, there was something else. It was awe. Pure, unadulterated awe.
She had known he was special. His mysterious appearance, his strange knowledge. She had pieced together that he was from another world, a world of metal birds and glowing rectangles. But it had all been abstract, a story.
This was proof.
This was not the work of a child, not even a prodigy. This was the work of an educated mind from a technologically advanced civilization, translated through the hands of a child. The principles of resistive heating, of regulated electrical flow… they were concepts completely outside the known sciences of Teyvat that deal with raw elemental energy, relatable, yes, but perhaps only in the forbidden ruins of Khaenri'ah. Yet this design held none of Khaenri'ah's arrogance or reckless power. It was simple. It was practical. It was… a gift. A solution to a common, human problem.
Slowly, she knelt down in front of Ren, the blueprint held carefully in her hands.
"Ren," she said, her voice soft and full of a new, profound gravity. "Do you understand what you have done?"
He shook his head, looking a little worried. "Is it a bad design?"
A rare, breathless laugh escaped Xianyun's lips. "No, little one. It is a perfect design. It is revolutionary." She placed a gentle hand on his messy, blue-streaked hair. "This is more than a clever invention. This is a spark. A spark that could bring warmth and comfort to every home in Liyue, from the grandest mansion to the humblest mountain hut."
She looked into his glowing azure eyes, and for the first time, she felt she was seeing past the adorable child. Her disciple was not just a lost boy she had found. He was a keeper of forgotten knowledge, a bringer of impossible ideas. And he was hers to protect, to nurture, to teach and perhaps, to even learn from.