Don't go to the moon, there are gods on the far side of the moon.

On the afternoon of the fifth day, the DreamNet listening terminal rang.

Lin Xiaoman heard it first.

"Someone's coming," she said.

"Who?"

"I don't know who it is," she said. "He didn't give his name. He was standing at the door."

Which gate?

"The door to the counterweight compartment," Xiaoman said, "He didn't go in; he just stood outside the door."

Everyone in the main control room stood up.

"Where is he?"

"He was outside the door," Xiaoman said. "He stood there for about two minutes, then knocked three times."

"Fast or slow again?"

"No," Xiaoman said, "this time it's—slowly, very slowly."

The people in the main control cabin looked at each other.

Fast, slow, fast, that's the sound of water from the west gate, that's the empty stove, that's which compartment.

Slowly, slowly, a new string begins.

"He didn't knock?" Lu Chenggong asked.

"He knocked," Xiaoman said, "but he left after he finished knocking."

"Retreat to where?"

"They retreated into the darkness."

Xu Wangshu did not rush to pursue.

She asked a more important question.

"When he knocked on the door, did the temperature in unit 31 even move?"

Qin Lan immediately went to investigate.

After finishing the investigation, she looked up, her expression not quite right.

"It moved," she said, "but in the opposite direction."

"What does 'reverse' mean?"

"Usually it's a matter of heating it up and then cooling it down," Qin Lan said. "When he knocked on the door, it cooled down for a moment and then heated up again."

"Cold first, then hot."

"right."

Xu Wangshu closed his eyes.

"That means," she said, "that this square was given way when he arrived."

"Give way?"

"It wasn't woken up by being knocked," Xu Wangshu said. "It was startled."

"Who are you trying to scare?"

"It scares itself."

No one responded.

Qin Lan watched that little bit of warmth for a long time.

"Normally, it's warm when it taps on its own," she said. "But when someone comes, it cools down first. What does that remind you of?"

"Like what?"

"When people from my hometown come back, the little warmth in the house is taken away by the door first," Qin Lan said.

"As soon as the door opens, the first thing to do is run away from the heat."

"Yes," she said. "It's not the heat that people take away; it's the heat that was already there when the door was opened."

Xu Wangshu did not respond to this analogy.

But she wrote those words down.

"Tell me exactly what you said," she said.

"Why do we have to remember this?"

"Because it shows that this grid recognizes the door," Xu Wangshu said. "A grid that recognizes the door doesn't need us to do anything. It knows on its own who has returned."

After a long pause, Lu Chenggong said something.

"Could it be," he said, "that the person who comes is the same one who scraped those words off back then?"

"Possibly."

"Then why did he come back?"

Xu Wangshu looked at the door that no one went in through.

"I'm coming back to find the door," she said. "The person who scratched off the markings is the one who remembers best where the door is."

"Should we stop him?"

"I won't stop you."

"Not stop them?"

"We'll leave a light on for him," Xu Wangshu said. "The door won't be fully open, and the light won't be on all the time. If he wants to come in, he'll have to walk in himself."

Qin Lan wrote this down.

"How should the record be written?"

"Write: On the fifth day, someone came to the door of the counterweight compartment, but did not enter. He knocked three times (slowly, very slowly), and left. His identity is unknown, so we did not pursue him."

"That's all?"

Xu Wangshu thought for a moment and added a sentence.

[No one is sent to guard the gate, nor is the gate closed. The gate has always been here.]

She finished writing and put down her pen.

"Slow down, slowly," she repeated softly.

"What do you mean?"

"I don't know," she said, "but one day, someone will tell us what those three strokes mean."

Xiaoman spoke up again from the listening end.

"He looked back once as he left," she said.

"Take a look at what?"

"I glanced in the direction of the grid," Xiaoman said. "Not at the door, but inside the door."

The main control room was silent for a few seconds.

"That further proves that he's here to familiarize himself with the place," Xu Wangshu said.

Why?

"The doorman watches the door," she said. "The person who watches the grids knows where each grid is."

"So this isn't his first time here."

"That's right," Xu Wangshu said. "Someone who has only been here once wouldn't know which wall the grid is behind."

Lu Chengong said in a low voice, "Then he had already acknowledged it once when he took the message away."

"He recognized me once, and then he came back again," Xu Wangshu said. "He's hesitating about whether or not to come back."

"What are you hesitating about?"

"I'm hesitating about whether to return that line of text."

After she finished speaking, she looked out the window at the darkness.

Black didn't move.

But this time, even Xiaoman said that the patch of black looked a little thinner than before.