The party bid farewell to the robot, Carly, immediately accelerating to the mysterious place. "See, I told you it had something to do with the camel scientist." Photon said to Natalie.

"But why did he design such a test? He needn't have tested us." Ranran said in bewilderment.

"I think he wanted us to experience the respect the residents on this planet have for green living." William wasn't so sure either. He also suspected that the camel scientist had set it up on purpose.

"Rebecca, we're almost there. But it's still plain around here, which is good for training. Shall we proceed to Lesson Two?" Photon looked around and said to Rebecca. She readily agreed.

Photon told William they were ready for training. "Very well, then you can take off." He said, "Come on, Rebecca! You are the future champion!"

Rebecca smiled shyly. The two planes rose quickly into the air.

"Come on! By completing this training, you will have completed half of the training! Today's lesson is -- the golden mean!"

"What?" Rebecca didn't understand what Photon meant.

"The so-called 'golden mean' is to take an average of many aspects, and not to emphasize any particular one. As for today's training, this word can be interpreted in a number of ways. The first is the height. I think I said in the last training that I advised you to fly as high as possible, but actually I forgot to say the rest of the content, and it's perfect to use this time to make up for it. Higher is not better."

"Huh? Why?"

"Because I looked at the structure of the planet's atmosphere. I found that if it was too high, the air would be thinner. This is not good for your engine's high power output, nor is it good for your maneuvering. So you have to take into account the rules of the race as well as your ceiling to find a place in low and high altitudes."

"Oh, I see! I'll try the altitude that suits me best." Replied Rebecca cheerfully. She then tried it at different altitudes and was surprised to find that the best altitude for her was exactly the one Photon said she would be at -- about 8,500 meters.

"That's just one aspect. Another one is speed. I know you're a fast plane, but even if you have been equipped with a more fuel-efficient engine, faster is not always better. You need to find a speed that ensures both lead and range. Try to refuel as little as possible, even in the air. Do the rules allow refueling in the air?"

"Yes, but there will be a speed limit of no more than 500 km/h." Said Rebecca.

"Well, that doesn't look as bad as I thought. Will it still be 100 hours this year?"

"Yes." Replied Rebecca.

"I see, that sounds easy enough." Photon said, "Now find your best speed for the race."

Rebecca sped up and watched her fuel consumption rate. She noticed a noticeable increase in fuel consumption at speeds above 700km/h.

"I know the solution." She said confidently.

Photon continued, "Next is the average of the lines and curves. Do the rules allow planes to fly in a certain space?"

"Certainly. If a plane is not flying in that space, its racing time will be reduced."

"So the route for this race has been announced, right?"

"Not yet, but the organizers have stated that in general there will not be many turns. It's just that the terrain in some areas is appropriate to test the cornering performances of airplanes."

"All right, then." Said Photon, "Now you see how much energy you lose when you turn. If we lose too much, we'll have to go back to Woodford's and continue to optimize for you."

Fortunately, the result was satisfactory: Rebecca could even become a fighter plane.

"Great! We don't need to go back." Screamed Rebecca.

"Yeah, it seems all you need to do is to master your cornering. If you can overtake a few more planes, that's even better."

"No problem, I can do it!"

Photon asked Rebecca to try again the energy retention for vertical maneuvers. The results were still satisfactory, though slightly worse than horizontal maneuvers.

"Do you know why that is?" Asked Photon, "That's because Woodford has perfectly managed to keep your wing load near the median level. That way, you're able to conserve as much energy as you can while maintaining enough maneuverability, especially horizontal. Don't think this race is nothing about energy -- energy might even help with sprinting."

"Oh, I see! Perhaps this is also part of the 'golden mean'?" Said Rebecca.

"You bet. My master, William, made this suggestion to Woodford in secret! He questioned him and learned that a plane with a median level of wing load was the easiest to win. That the wing load is too high will result in poor maneuverability, which will easily lead to the loss of advantage at corners; being too low doesn't allow a plane to conserve as much energy, and it wastes fuel as it accelerates or climbs."

"Wow. I want to thank him after the race."

"Well, that's the end of our lesson. Why don't you give a brief introduction to the others?"

They landed and came to a slow halt in front of William. Rebecca gave a brief introduction. "What impressed me most was actually the average of speed and range." She said, "I found that in my previous tryouts where I didn't win, it was because the speed advantages of champions were too obvious. However, I also recall that it took a long time for them to refuel in the air, during which time I could almost catch up with them. But then they picked up speed and took a big lead."

"Well, that's an important point." Ranran nodded, "Will that huge speed advantage not be allowed in the main draw?"

"The organizers haven't announced it yet, but they don't seem to want that to happen right now." Rebecca said. Then an unread email appeared on William's computer. "It's from Woodford!" He cried in surprise.

Natalie read.

Dear William,

How are you doing? Your little robots told me you were helping Rebecca with her training. It is a good idea, because she hasn't flown for two years. But the robots also told me that she was learning quickly, which was surprising because I had the impression that it took her a while to get used to it.

I just got some good news from the organizers of the race. They have announced that the selection process for the last two years of the tryouts was too difficult. The reason is that some competitors had too obvious speed advantages, which made it difficult for others to overtake them. The statement said this year's main draw would eliminate such excessive leads.

I'm sure Rebecca will be overjoyed when she sees this email! Anyway, thank you for your recognition of my level of refit. And I wish Rebecca the champion this year!

Woodford

"Great!" Rebecca jumped up excitedly, "I can finally stand on the top of the podium again!"

"Hail for the better days to come! Yeah!" The roar of the party was deafening and rang through the air.

The two mother stars sank, and the sky grew dim. Their afterglow was unreservedly shed by them. Like a steady stream of waterfalls, the light fell on the purple planet. Though the party was still in the green plains, they would soon reach the purple plains. Rebecca looked at the two mother stars that were about to disappear below the horizon and smiled. As if by magic, she saw the prospect of winning again. She stood on the podium, surrounded by a flood of cheers, with colorful ribbons flying.

"I know that day will come." Rebecca said to everyone, "I have the golden mean, and that's a very important foundation. And most of all, I believe I can win."

"Come on, Rebecca! We are always there for you!" Ranran said, while the others cheered loudly again.

The two mother stars completely hided their faces as darkness fell. A cool breeze blew, and the grass on the ground danced gracefully. The party in the night moved on quickly. They walked tirelessly, day and night, in order to reach the next place as quickly as possible. Now the mysterious place on the map came nearer and nearer to them. But to William's horror, a note fell from the air on his head.

He opened it and saw, once again, the familiar writing:

There's no way you're going home, no way!

"What's the matter? Who the hell is that man? Why does he want so much to stop me going back?" A series of questions exploded in his mind. Then he felt a terrible pain. It was as if a hand were gripping his head and tearing it mercilessly apart. He said nothing, laid his hands on his head, and bore the pain alone. Seconds later, the pain was gone.

"Well, let it go. Anyway, I will be able to go home." William told himself, "We'd better hurry. The mysterious place is almost here!"

They could already see a huge hole with some fruit painted on it. "Why are these signs? What does that mean?" Photon and Natalie asked, puzzled.

"I don't know what they mean." William said, "Let's go in and see them."

"Yes, Master. Be safe."

"OK." William clutched his laser gun and knocked on the door together with Ranran.