The sunlight shone on the metal sheet, reflecting a blinding light.

Chen Hao raised his hand to block it, said nothing, and simply stuffed the wrench into his tool bag. He stood there, looking at the installation on the roof, the pile of assembled metal plates and pipes lying quietly, like a metal beetle basking in the sun.

He even gave it a name yesterday.

Call him "old buddy".

Now the water isn't cooperating, the temperature won't rise.

He crouched down and reached out to touch the pipe near the water outlet, pulling his fingertips back as soon as they touched it. The surface was hot, but the inside was cold. No hot water came out; only slightly heated, cold water flowed out.

“Thirty-three degrees.” Nana stood beside her, scanning the data with her optical glasses. “Nine degrees higher than the ambient temperature, but not high enough for bathing.”

"I know what temperature is comfortable for a shower," Chen Hao said in a low voice. "Forty degrees or higher is comfortable, anything lower is torture."

He stood up, walked around to the back of the device, and stared at the spliced ​​metal panels. The black paint was peeling, and the edges were curled up, like wallpaper that had been baked by the sun for too long.

"You're saying it doesn't absorb enough heat?" he asked.

“There are two main problems,” Nana replied. “First, the angle is wrong; the sunlight is at an angle, resulting in low absorption efficiency. Second, the water flows too fast and is discharged before it can be heated.”

"Then adjust the angle and turn down the water flow."

"It has already been suggested."

"Then why didn't you explain it clearly just now?"

"You are tightening a screw."

"I'm listening now, aren't I?"

He walked back to the support frame, grabbed the bolt on the fixing plate, and tried to pull it off. The screw wouldn't budge. He tried again, his arm tense, his face flushed, but it still wouldn't loosen.

“It’s too tight,” he said.

“The metal expands when heated, causing the hole to shift.” Nana stepped forward. “I’ll stabilize the structure, you operate it.”

She used the robotic arm to hold the bracket base in place, her metal fingers wedging into the grooves to firmly press it down. Chen Hao took out a wrench, realigned it with the bolt, and this time it loosened with a gentle twist.

"You should have done this sooner," he muttered.

"You need to feel involved."

"I thought you were worried I'd slack off."

Piece by piece, they raised the metal plates, adjusted the angle, and tried to align them with the sun. Once finished, Chen Hao stepped back to look at them and thought they looked better.

“Now it looks like a proper water heater,” he said.

"The appearance does not affect the function."

"But I find it pleasing to the eye."

Next, the throttle valve was replaced. The original valve was a temporary modification using old parts, and the switch only had two states: open and half-open. Nana found a small knob with markings in the spare parts and replaced it.

“The water flow rate can be fine-tuned,” she said. “An initial setting of 0.8 liters per minute is recommended.”

"It sounds like a prescription."

"This is the optimal value."

Chen Hao turned the valve, and the water flow immediately became thinner, the dripping sound slowing down. He looked at the outlet, waiting for the temperature to change.

Ten minutes later, the end of the pipe began to get slightly warm.

Twenty minutes later, he reached out to test it and found it was a little hotter than before.

"There's potential." He grinned.

Ten minutes later, the smile slowly faded.

"Why did it stop again?" He touched the pipes. "It was heating up just now, why did it suddenly stop?"

"Thermal equilibrium has been reached." Nana scanned the internal temperature. "The input of heat equals the loss of heat, so it cannot continue to accumulate."

"Does that mean you can never be promoted again?"

"Unless the structure is changed or the heating time is extended."

"That won't do. I can't wait five hours every day to take a shower."

He stared at the device, his brow furrowed. Sweat trickled down his temples, gathering into a drop on his chin before falling onto the wooden ladder at his feet.

“Revise it again,” he said.

"Three schemes have been recorded, but the current material only supports one of them."

Which one?

"Add water storage tanks to form a circulating preheating system."

How big do you want it?

"At least ten liters in capacity."

"We don't have any."

"Two five-liter buckets can be connected in series."

"Where are the connectors? Where are the seals? Where are the brackets?"

"Requires to be made by hand."

“That means there isn’t one.” He waved his hand. “It’s too much trouble; we can’t finish it today.”

He looked down at the thin tube in his hand, and suddenly a thought struck him. "Wait, did we make a mistake?"

Please explain.

"These two pipes are connected in parallel now, right? The water is split in two and flows at the same time, which means the distance is halved."

"correct."

"What if we change it to a series connection? Let the water go through one section first, and then into the second section. The distance is longer, and the heating time is also longer."

"It's theoretically feasible, but the interface needs to be redone."

"Just do it."

The two men started disassembling the pipe. Chen Hao pulled off the connector and found that the rubber gasket was old and broke easily. He tore off a piece of cloth and wrapped it around the threads to barely seal it.

"Temporary solution," he said.

"Risk leakage".

"Fix it if it leaks."

After the new layout was installed, the entire pipeline became an S-shaped bend, and the water flow path was significantly lengthened. They re-secured the supports and checked each joint to ensure that nothing was loose.

The third test begins.

The sunlight remained bright, shining on the sloping metal plate. Water flowed slowly into the first section of the pipe, moving forward along its winding path.

Chen Hao sat on the edge of the roof, leaning against the chimney, his legs dangling in the air. He held the discarded thin tube in his hand, straightening it and bending it back down repeatedly.

Within ten minutes, the temperature rose significantly.

Fifteen minutes later, the temperature was close to thirty-five degrees Celsius.

"Soon," he murmured.

After twenty minutes, the thermometer stopped at 36 degrees Celsius and stopped rising.

He stared at the reading, blinked, and blinked again.

"Just four degrees short?" His voice was a little hoarse. "Four degrees!"

"The current maximum theoretical output is 37 degrees Celsius," Nana said, "but it is limited by the heat collection area and the rate of heat dissipation from the environment."

"So, we've reached the end?"

"Yes, given the existing structure."

Chen Hao didn't move or speak. He looked down at the thin tube in his hand and bent it into a crooked Z-shape.

The sound of wind whistling through the roof came from afar, causing the sheet metal to vibrate slightly.

He suddenly smiled.

“Do you know what the worst thing is?” he asked.

Please explain.

“I thought we were making progress. Grinding iron plates, building scaffolds, connecting water pipes, changing angles, adjusting water flow, changing layout... every step seemed to be working properly, as if we were getting closer and closer to success. But what happened? We spent the whole morning just to prove one thing—this thing was never going to work.”

"The process is valuable."

"But I need hot water."

"This will help us avoid making the same mistake next time."

"But I still can't take a shower."

He threw the thin tube on the ground and looked up at the sky. The sun was already high overhead, and the rays were shining down, making his scalp feel hot.

"Wait a little longer," he said. "The sun hasn't reached its highest point yet."

"The current light intensity has reached its peak."

"Maybe it's just a few minutes' difference."

He sat motionless, his eyes fixed on the water outlet. Water was still dripping there, slowly and evenly, the temperature remaining consistently around thirty-six degrees Celsius.

Nana stood beside him, the camera slightly turned toward him, the optical lens continuously updating the data.

"I recommend stopping this test," she said. "Record the parameters and reassess tomorrow."

"I don't want to admit defeat."

"This is not a matter of winning or losing."

"That's how it is for me."

He raised his hand to shield his eyes from the sunlight, squinting at the patch of reflected light. The spot of light landed in the center of the device, swaying as if mocking something.

"Do you think we're taking this too seriously?" he suddenly asked.

"All operations are based on reasonable deductions."

“I’m not talking about the technology. I mean… the two of us are here, testing and modifying, making it seem like we can actually build a solar water heater. But what is it really? A pile of scrap metal, a few broken buckets and pipes, not even a decent sealing ring. And you’re still giving me a serious lecture on heat balance.”

"Knowledge can be applied regardless of conditions."

"Results can be scored."

He looked down at his hands, his palms covered in calluses and abrasions, black ash embedded in his fingernails. He remembered feeling quite capable just yesterday, able to climb onto rooftops to work, to hand-polish iron plates, and to solve problems with robots.

I just feel tired right now.

"What do you think I'm after?" he asked.

"Improve living conditions."

"That's right. I want to take a shower, drink hot water, and live a normal life. But the reality is, I can't even warm myself up."

"That doesn't mean it's impossible."

"But I can't do it today."

"yes."

"You are honest."

He took a deep breath and slowly exhaled. Sweat soaked his collar and clung to his back, feeling cool.

“Try one last time,” he said.

"How do we do it?"

“I don’t know,” he shook his head, “but I can’t just leave like this.”

He stood up, walked to the device, and reached out to touch every inch of the pipes and every joint. He lay down and looked up at the shadow of the support frame, watching how the sunlight fell on the metal plate.

"Is the light... not enough to cover the entire area?" he suddenly asked.

"The irradiation coverage rate is 82 percent."

"What about the rest?"

"Obstructed by the support frame."

"Could you move it a little?"

"Structural load-bearing limitations".

He stopped talking, squatted there, and stared at that small patch of shadow.

The sun was high in the sky, the sheet metal was scorching hot, the water was still flowing, but the heat did not come.

Chen Hao reached out and grasped the water outlet, feeling the last stream of water slide through his fingertips.

The temperature remained unchanged.