Then, just three minutes later, William was at the recycling center. "Look," Ranran said, pointing to a large circular sign, "here we are. Hurry up and get in."

The party rushed through the door. Waiting for them was a robot of the same type as Hans had, but this was a girl. She said, "Hello! You must be William."

"Yes. How do you know my name?"

"Mr Bush, whom you just visited, has told me that you will be here soon. Come with me." Carly was about to lead them away, but she stopped and said, "Wait, you must be tired. Why don't you go to the break room first? I will take you there when you have got your strength back."

If the robot did not say, the party would not feel tired; hearing her words, everyone felt a little exhausted. So the robot took them to a huge lounge with four large and comfortable double beds. Airplanes and the racing car didn't need to sleep in beds -- they didn't need them anyway, and there was still plenty of room left.

"See that button by the bed?" Carly asked William, "If you're awake, just press this, and I'll be right over."

"Okay, thank you very much." William thanked the robot and she went away, leaving the weary party to rest for five hours. Another day had passed.

William was puzzled that his spirit was not dropped, despite his recent lack of rest. What he didn't know was that on the night he slept in the mushroom pianist's big room, a small robot in the backpack had secretly implanted a program that automatically adjusted his body clock. That was why he didn't feel too tired.

The next morning, there was a clear sky, while the warm rays of the two mother stars fell lightly on the sleeping party. Perhaps awakened by it, William got up and pressed the button. Carly came as soon as she received the signal. "Are you ready?" She asked, "I can take you there anytime."

"Where?" Photon asked. "You'll soon find out." Carly deliberately kept them guessing, leading them to a door. William noticed a lightning sign on the door which they thought was very familiar. Then, as if the sign had magic, William saw images of them sleepwalking in the purple room on the previous planet.

"I know! We've got some lithium batteries, and the hint says if we see a lightning sign, we can use them!" So he took out those batteries and put them on the lightning sign. Then the sign gave off a faint light. The door opened.

William was stunned by what he saw. A large number of used batteries were piled up inside, waiting for further recycling. Carly was usually responsible for recycling them, but this time she deliberately let them try it out. In terms of why that was, she said that they would have to wait and not be told until they were done.

"These are used batteries from all over the planet." Carly said, "They're made of different materials and they have different properties, but the main categories are all here." she handed them a disk and explained, "The file 'Categories.xlsx' lists the types of these used batteries; 'Treating methods.xlsx' provides the most feasible approaches, but they are not complete. Now, all you need to do is to fill in the missing items and match the types of battery with the treating methods so that each type of battery can be recycled in the most efficient way. This is also a kind of respect for resources. If you're done, press Ctrl and F1 and I'll be right over. Good luck getting it done soon!"

"OK." William said. "Then I'll go and attend to my other business. I'll see you later." The robot moved away.

William took a few minutes to think about other things. "I feel it has much to do with the camel scientist, just like Hans the engineer and Bush the bookseller." He thought thoughtfully, "That they both know him shows that he is really famous and powerful. In addition, the engineer invited us to visit his windmills and tube train, Bush needed us to help him design more environmentally friendly books, and now Carly is giving us the chance to experience battery recycling... I guess it was all arranged by the camel scientist. Perhaps he has set all this up to test us? Or he wants to give us some techniques? Well, it's no use thinking about it now. I'll wait till I see him in person."

"Master, are you all right?" Photon, Rebecca and Natalie watched him thinking thoughtfully. They asked, a little worried.

"Oh, I'm all right." William pretended to be relaxed. But they felt he was hiding something.

Ranran told William, "We have just opened the sheet and found that there are many types of batteries, but most of them are rechargeable batteries or fuel cells and the proportion of primary batteries is very small, so we don't expect it to be too difficult."

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"That's great. Let's think about feasible ways." Said William to himself. When he finished saying, he opened "Treating methods.xlsx" and saw the words "disassembling" "acid leaching" and "filtration". "Other rechargeable batteries do not require acid leaching, so this is the way primary batteries should be treated." He thought.

"I guess acid leaching is used to recycle the zinc canister." Photon said, "It will be oxidized to salts and then it can be reused."

William nodded, "That makes sense. Filtration should refer to the treatment of the composition in the pasty electrolyte. Other elements, such as chlorine and manganese, can still be made into other types of drugs that can be used."

"Then let's complete it quickly!" Ranran said. So they completed the treating methods of primary batteries in three minutes.

"Then there are rechargeable batteries. They're made of graphite, silicon particles, lithium, sodium, potassium, and their phosphates." Said Natalie, "Electrolytes are the safer organic matter."

"All right." William began to think about solutions. He saw the words "dissolution" and "electrolysis" in the document. "That's it." He said firmly, "Graphite and silicon particles are hard to dissolve, so they're screened straight out. The salts in those electrolytes can be extracted by dissolution and crystallization, while the organic matter can be utilized by oxidation, microbial treatment, etc. As for the base metals, I think electrolysis is a good idea. But they should be electrolyzed in their fusion state rather than in an aqueous solution, otherwise they cannot discharge. So what we can do is that we can oxidize the remaining free base metals into salts, separate the salts and heat them until they melt, then electrolyze them." He filled in the blanks as he pondered.

"All right, finally the fuel cells." William said, cheerfully sorting out the newly filled document, "This is easy because most of the electrode materials that make up the main bodies of the batteries are inert electrodes, such as platinum and graphite. These are not difficult to deal with. Even with the active electrodes, we can refer to the solutions just done, and the electrolytes are all similar. So I'll be done soon." Sure enough, it took him even less time to complete the procedure than that of rechargeable batteries.

"Ta-dah! We're done, Carly!" William saved the file and pressed the shortcut key. Carly came in to check.

"Wonderful! You are as smart as a whip!" She exclaimed, "All correct, and quite quickly! Amazing! Come on, I'll show you your new gifts!"

"New gifts? What new gifts?" Photon and Natalie muttered as they walked, "Even better than the batteries used to open this door?"

"Sure! Look, there it is!" Carly pointed to a small box.

Ranran went up and looked at it carefully, but did not find any flaws. So she asked, "Is it a box? It doesn't seem to do anything else."

"You've been tricked sure enough, Ms Ranran!" Carly laughed, "It turns out it's very well disguised. It is a fusion cell that relies on protium, deuterium and tritium, which are isotopes. Give it a try! It is now full of nuclear fuel, but still exceptionally light. It takes almost no energy to carry it."

Carly was right. "Wow, how was this done?" Ranran asked pleasantly surprised. Carly answered, "In addition to carbon fiber composites, it also uses a lighter material, silica aerogel."

Some of the materials he had been exposed to at Hans' and Bush's flashed through William's mind. He felt more and more that this planet was very much like his home, Earth. "Is it just a coincidence?" He thought excitedly, "Or did someone do it on purpose?" He soon ruled out the second hypothesis. He said, "My universe is so big that it is absurd to say that it was designed by somebody, let alone that he knows about other universes."

But it was also quite probable that the hypothesis he ruled out was the truth. He had taken that into account, of course, but so far he had still believed his first hypothesis.

"It could beat chemical batteries." Said Carly, "But even the smallest fusion cells are bigger than the usual micro batteries. They are currently only available on portable phones and larger devices."

William heard the unfamiliar term portable phones. He looked it up on his computer and learned that it was no different than a smartphone on Earth.

"It seems not easy to make them smaller." Photon said to William, "I guess only fuel cells can serve as smaller batteries."

"It's extremely difficult to get it to work on smaller devices." Rebecca said.

"Yes, exactly." Said Carly reluctantly, "There's a scientist who's trying to figure out how to miniaturize fusion cells. Do you know the camel scientist? He's the one doing the research."

"We've heard of him, but we haven't seen him yet." Replied Rebecca, "So many people know him! He seems really famous."

"That's right. He is now known to almost the entire planet. You're aliens, so it's normal not to know, ha ha."

"We're planning to visit him." Natalie said, "Excuse me, do you know how to get there? I'm afraid our maps aren't accurate enough."

"You're right to be worried, Natalie." Said Carly, "There's no map showing where a camel sign is, but once you've found it, it's easy to find the camel scientist's quarters. I hear that Mr Bush, the owner of Leo Bookstore, has shown you directions. Is that true?"

"Yes, except that he mentioned a mysterious place." William replied. "We don't know what it is." Natalie took his words.

"Actually I do, but it wouldn't be interesting if I told you now." Carly was again keeping them guessing, "All I can tell you is that it's safe. Your lives will not be in danger. The rest is up to you to explore. In this way, the fun belongs to you, not to me. It wouldn't be any fun if I just told you what's in there."

"Well, thanks for your help!" Ranran bowed and said.

"You must hurry to that mysterious place." Said Carly, "Days are short on this planet, and you must know that already. In addition, the reason you have this test is that the camel scientist wants to test you. You guys are really good enough to pass the test and get such a good battery. I admire you very much! You'll find out more about him when you visit the camel scientist. Bon voyage!"