"Only once, I sent a large stone of more than 40 catties to the sky." Huang Han said excitedly.

"How high did you fly?" Yu Ren's question was simple and direct.

Huang Han's voice was much quieter at this time. "About a dozen feet high, and then it fell."

"Rare, rare!" Yu Ren praised from the bottom of his heart.

"In that case, then how dare you make that bet with Fat Xu just now?" Yu Ren asked puzzled.

"I've recently improved the four fireworks I used in the first few trials and put more gunpowder. I believe that these four things must be much more powerful than the last time. So I have faith that they can send me to heaven. Huang Han said.

Yu Ren nodded. "I have a question I'd love to ask you. Do any of you know why something that we so easily sent to heaven fell so quickly? ”

Cao An, who was on the side, couldn't help but laugh when he heard this. "Childe, are you confused? If this thing doesn't fall to the ground, will it still fly into the sky? ”

Yu Ren gently patted Cao An's shoulder. "You're really smart, and you really have something that doesn't fall to the ground, but to the sky!"

"What's that?" Everyone asked.

"Kong Ming lantern!"

"But it's going to fall eventually!" Huang Wuyi interjected. "All the Kong Ming lanterns I saw eventually fell off."

Yu Ren nodded. "You're quite right. So, have any of you ever wondered why Kong Ming lanterns flew into the sky on their own, and why, did they end up falling? ”

This time, no one spoke to anyone again. After a long while, Cao An asked Yu Ren puzzled, "Childe, what do we want to do with this?" ”

Huang Han raised his head and answered Cao An's question for Yu Ren. "Because, if we understand this, can we come up with a way to make Kong Minglan never come down. In addition, can we also make a big Kong Ming lantern and let it never stop rising and carry us to the moon? ”

Yu Ren had to admit that this Huang Han was really a very talented person. Although he didn't say everything right, at least he had grasped some key points under Yu Ren's reminder.

Yu Ren is like a physics teacher, teaching the ordinary children in front of him step by step.

"You're right. So, have you figured out the reason why Kong Ming lanterns can rise into the sky? ”

All three of them stopped talking. Huang Han's brows were even tighter.

After a long while, the three of them still seemed to have no results. Yu Ren didn't want them to waste any more time, so he gave them a detailed explanation of what air is, what density is, and the density of air.

"The reason why the Kong Ming lantern rises is because the air inside is heated and expands, and the density is less than the surrounding air, so it rises. This is where buoyancy comes in. When the contents of the lamp are burned, the air inside the lamp gradually cools down, and they become the same density as the surrounding air. As a result, the Kong Ming lantern fell from the sky like anything else. This is the action of gravity. ”

Seeing that the three of them were still the same, they didn't understand anything. Yu Ren had to explain further.

"To put it simply, I'll use water as an example! If you put something heavier than water, such as a stone, into the water, it will sink to the bottom of the water immediately. If you put something lighter than water, such as cotton. Even if you put it under the water, it will immediately float up from under the water. This is the role of buoyancy. In the same way, air is also buoyant. ”

When Huang Han heard this, he seemed to have some understanding. For the first time, a knowing smile appeared on his face. But Huang Wu and Cao An, as soon as they saw their expressions, knew that they still had nothing.

But Yu Ren didn't want to explain more to them on this issue. Because he knew that it would take too long to explain everything to them. So he said to Huang Han, "Now, have you thought of any better way to send people to heaven?" ”

"I think we can really make a big Kong Ming lantern and let people sit in it, so that they can go to heaven." Huang Han Dao.

Yu Ren gently patted Huang Han's shoulder. "The children can be taught!"

Just do it. Huang Han thought of a way, so he planned to act immediately. However, Yu Ren advised him to draw drawings first, and then make them according to the drawings. In this way, errors can be reduced. Huang Han listened to Yu Ren's suggestion and asked Huang Wu to go to the city to buy the four treasures of the study. But Yu Ren picked up a carpenter's ink fountain that was thrown on the ground, and said that he didn't need to buy it, but used this first. This one is a bit easier to use. It just so happens that the ruler is also ready-made.

So, under the auspices of Yu Ren, this group of people began their new flying plan: to make a big Kong Ming lantern - a hot air balloon.

The production diagram of the hot air balloon is relatively easy to draw, and the most important thing is what material to use. This really made everyone brainstorm. In the end, at Yu Ren's suggestion, they decided to make a small hot air balloon with bamboo baskets, paper, and bamboo chips.

Seeing that it was already noon, everyone hurriedly split up and went down the mountain to prepare materials.

This is because the production is a relatively simple prototype, and the size of the production is relatively small. As a result, by dark, they were basically ready. This is an enlarged Kong Ming lantern. Underneath is a bamboo basket with a brazier and a large round ball made of simple thick paper on top.

When everything was done, there was hope. Yu Ren lit the brazier under everyone's expectant gaze. Then, their eight pairs of eyes were staring at the bamboo basket.

Time passed by a minute, and everyone's mood was extremely anxious. After about a quarter of an hour, I finally saw the bamboo basket start to beat. At first, it only jumped lightly, then slowly floated off the ground, and gradually ascended.

At this time, everyone held their breath, looking at this Kong Ming lantern, which had been magnified several times, full of their hopes and dreams, and was flying towards success step by step.

When the hot air balloon flew overhead, Cao An finally couldn't help but shout, "Ah! It's a success! ”。 Huang Wu also shouted, and the two hugged each other tightly after shouting. Huang Han on the side stood still and did not move, he felt that something wet was flowing in his eyes. A dream for many years! At this moment, he can finally clearly feel that he is getting closer to his dream step by step. Dreams are no longer illusory castles in the sky!

Only Yu Ren was calm, because he was thinking about what to choose as fuel and what cloth to make the hot air balloon the most appropriate to use to continue to enlarge the hot air balloon. In the production, it is necessary to take into account both the weight and make it as light as possible, but also the sturdiness and impermeability.

Just when everyone was immersed in joy, suddenly a shout came from behind them, "Hurry, hurry." It's on, it's on! ”

Several people heard the cry and hurriedly looked up together. Sure enough, I saw that the hot air balloon that had just risen overhead was already on fire. At first, only the bamboo pieces connecting the top and bottom caught fire, but soon, the fire burned to the paper on it. As a result, the hot air balloon, which had only been in the air for more than a minute, suddenly fell from the sky within a few seconds, fell into the mountain stream next to it, and disappeared.

Seeing this, Cao An rushed to the side of the mountain stream and was about to go down to find it.

"Forget it." Yu Ren stopped him. "This is just a test piece for us. If you can achieve such an effect, you can do it. I didn't expect it to fly far. But this result clearly tells us that the enlarged and manned Kong Ming lantern in our plan will definitely succeed. ”

Originally, Cao An and Huang Wu both looked depressed when they saw the hot air balloon on fire, but after listening to Yu Ren's words, they both cheered up again.

Yu Ren walked up to Huang Han, who had been staring at the hot air balloon falling into the mountain stream, but never said a word, and gently patted him on the shoulder. "Rest assured, we'll make better hot air balloons soon."

Huang Han looked up at Yu Ren, and then nodded firmly.

Yu Ren comforted the emotions of several people, and then walked towards the person who had been hiding in the dark until the hot air balloon caught fire, and finally couldn't help but scream.

In fact, Yu Ren didn't need to look at it, he already knew who this person was. Because, from his voice, Yu Ren had already guessed it.