C90 (1/2)

The old lady sighed and said, ”I was very worried about him at the time, but I hate him now. Why didn't I chase him then? I waited in the house until midnight, but still the old man didn't come back. Only then did I get anxious, so I took the umbrella and went out to look for it. I looked all the way, calling out the old man's name. But in the middle of the night, with no one around, how could anyone agree?

I don't know how long I searched, but I finally came to a road. The sewer of this road was broken, and it had accumulated a lot of water as deep as my ankles. It was on this road that I suddenly discovered the old man's umbrella. Yes, it was the umbrella he had brought with him when he left the house. There was a hole in the umbrella. But when I looked around, there was no sign of the old man. I shouted, my throat cracking, but no one answered.

”I don't know how long I shouted before I noticed that the well entrance to the sewer beside the highway was actually open …”

Saying this, the old granny wiped away her tears again and said, ”Speaking of which, you should already know that the old man fell into the sewer during the night. When I found him, he was already cold all over.

If my son hadn't insisted on selling the ancestral home, this old man wouldn't have fallen out with him and this tragedy wouldn't have happened. My son also regretted it now, but what could he do about it? The ancestral home had already been sold, so we had no choice but to move our entire family here.

I think the old man must still be resenting his son, which is why he came to me. I can feel it, this old man, right here in this room … ”

At this point, the old woman looked around and shouted, ”Old man, he is our son, please spare him!”

Benoit and I listened with rapt attention and felt sorry for the old lady.

”You don't have to worry, grandma. Actually, that wasn't a ghost pushing the door just now. I'll demonstrate it to you now and let you know the principle behind it.”

With that, Bai Nanxin stood up and walked to the door leading to the balcony. She held her hand and said, ”Granny, watch carefully. When I open this door, the door to the living room will definitely open.”

As she spoke, she gently twisted her slender hand and gave it a push …

Creak …

It was just as Bai Nuxin had said, the living room door had actually been opened by itself!

What was going on?

The grandma and I were stunned. Why did the door to the balcony open as well?

”Actually, the principle is very simple. This room is very sealed, it's very hot on the outside, but cold air is on the inside. The cold air is equivalent to putting pressure on the inside of the room, and when the door to the balcony is opened, it's equivalent to letting air out from the outside of this room, and because of the temperature difference, the cold air inside the room will quickly gush out.”

At this point, Bai Nanxin walked to the front door of the living room, closed it, and continued, ”The air pressure inside and outside of the room will decrease rapidly as the air pressure rises, forming a pressure difference between the inside and the outside, just like when a ball is suddenly punctured. The air pressure will quickly leak out and the ball will dry up, and this room, the living room's door, will be pushed open by the air pressure.”

Hearing this, I had a question. ”That's not right. Once the door closes, the lock will close. Even if you kick it hard, it might not open, let alone if it's just a difference in air pressure.”

The grandma was confused after hearing Bai Nanxin's explanation. She didn't know what the difference in air pressure was. To her, it was like a third-order function that was as hard to understand as a primary school student.

The old woman said, ”Little girl, although I don't know what you're talking about, I don't agree with you. As the young man said, the door has already been bolted shut. Unless someone twists the handle, it's impossible for the door to open.”

I nodded.

”Yes, your words make a lot of sense, but what if the lock on the door breaks?”

As she spoke, Bai Nanxin slammed the door shut and then pulled it open. Throughout the entire process, she had not twisted the door handle at all!

I was stunned for a moment. So it was like this …

Bai Nuxin explained, ”It's your habit, Granny, to open and close the door and close it. Naturally, you ignore the minute changes on the door, but the spring on the lock tongue is broken, and once the door is closed, the lock tongue will be pressed in by the door frame and won't be able to bounce out. However, if the door is locked with a key, the lock will pop out when the tongue is twisted, so the door can still be locked, which makes it difficult for you to detect the change in the room. ”

The grandma stared at the door for a long time before suddenly laughing out loud. ”You're still a little girl, your eyes are sharp. This old woman really didn't notice the lock on the door.”

”So that's how it is …” I couldn't help but have a new level of respect for Bai Nanxin. I didn't expect her to have such great observation and inference skills. She was simply a female detective.

Then, I couldn't help but find it funny. So it turns out that pushing a door is such a simple principle? What's even funnier is that I actually thought that it was a ghost who pushed the door open …

I had learned all these physics in high school, but now I was ”possessed” by a preconceived belief that there really was something wrong.

”That's why you don't have to worry, Granny. There are no ghosts knocking at the door. There are no ghosts in this world. Everything can be explained by science.”