Song Huiqin followed the old man into the laboratory and found that the laboratory was like a movie theater, although it was underground, it was not as eerie as imagined. There is a stage at the front of the laboratory, and a large screen on the stage is covered with dense small white dots, these white dots are light and dark, like a zebra crossing on the road. There is a small hole in the back of the lab with a projector in the hole. Oddly enough, there is also a bezel between the projector and the movie screen, and there are two slender slits in the middle of the bezel.

Song Huiqin felt very strange: this baffle is blocked between the projector and the screen, how can the movie be played? Where do the audience sit? If you sit in front of the baffle, the baffle blocks the light from the projector, where is the image on the screen? If you sit behind the baffle, the projector is projected on the baffle, and the picture is too small, why make two thin slits in the middle of the baffle? Doesn't it affect the viewing effect of the film?

"Uncle, is this lab used to play movies?" Song Huiqin asked.

"The main function of our lab is to conduct scientific research, and showing movies is just an additional function. We experimented with the fact that when we were resting, we could remove the middle baffle and we could play the movie. The old man said.

"It turned out to be a multipurpose cinema. What is this baffle in the middle used for? Song Huiqin asked.

"This baffle is used as a fork in the road. Let's say a person comes out of the projector, goes in a straight line, comes to this baffle, and sees that there are two slits on it, which means that this is a fork in the road, and this person has to continue to move forward to reach the screen, how many paths does he have? The old man asked.

"How does this man move forward in the air? Will he fly? Song Huiqin asked.

"Suppose there is a wire rope in the air, he walks on it, and he encounters a baffle, how should he walk?" The old man asked.

"He can go through the left slit or the right slit — if he's thin enough to get through the slit." Song Huiqin said.

"Can he pass through both seams at the same time?" The old man asked.

"Are you saying that this person walks both the left and the right seams at the same time? Will he do the doppelganger? Song Huiqin asked.

"We just don't know if he can do the doppelganger, so we're going to do this experiment." The old man said.

"Teacher, the experiment is ready, can you start now?" The assistant next to the projector asked.

"Erase the white dots from the screen first, don't affect the results of our experiments, and then fire the bullet!" The old man pulled Song Huiqin to the side to avoid being accidentally injured by bullets.

The assistant pressed the remote control, the white dot on the screen disappeared, and then pressed the button, the projector turned into a pistol, constantly sending bullets to the middle baffle, and Song Huiqin heard the sound of gunshots.

"Uncle, is this the beginning of the shootout movie? How can there be a movie with real bullets? Song Huiqin asked.

"It's an experiment, you learn first."

Some of the bullets were bounced back by the baffle, and some of the bullets passed through the doublets and hit the movie screen, leaving bullet holes in the screen. The bullets kept firing, leaving two parallel bullet holes in the screen behind the double slit.

"Son, can you explain the current experimental phenomenon?" The old man asked.

"It's easy! The bullet was fired at the baffle, and the bullet that was not aimed at the slit bounced back, and only the bullet aimed at the slit could pass through, and after passing through, it hit the screen, of course, forming two bullet holes parallel to the slit. Song Huiqin said.

"Can you form three, four, five, six, seven...... What about the bullet holes of the above - ? If the screen is wide enough," the old man asked.

"Definitely not, unless there are three, four, five, six, seven ...... slits"

"There are only two slits, not many." The old man said.

"Are you testing my brain teasers? Your question seems to be: out of ten birds in the tree, shoot and kill one, how many are left? Song Huiqin asked.

"I'm not talking about brain teasers, I'm talking about real experimental conditions."

"That's you're playing magic." Song Huiqin said.

"It's not magic. The subjects behind us will get smaller and smaller until such a miracle happens. The old man said.

"Impossible, right? How can there be multiple bullet marks in two slits? It's not scientific! Song Huiqin said suspiciously.

After the bullet was fired, the old man asked his assistant to change to iron sesame seeds to do the experiment. The assistant presses the remote control, the movie screen has an automatic repair function, and the bullet holes on it disappear automatically. Another man really brought out a pot of iron sesame seeds for the assistant to experiment, and this man was one of the palanquin bearers who carried the sedan chair for Song Huiqin. The assistant selects an image in the computer system, and the pistol that the projector turns into is deformed again, turning into a nozzle with a small aperture, and the assistant puts iron sesame seeds into it, presses the button, and the nozzle continuously sprays iron sesame seeds to the front baffle, and the iron sesame seeds that pass through the slit are embedded in the screen, and two parallel sesame cakes corresponding to the slit are formed.

"We have completed the experiment with bullets and iron sesame seeds, and the results are the same. We continue to experiment with an intangible object, so pay attention to it. The old man said.

The assistant took a bulging plastic bag, which contained an invisible thing, the wind, which was collected from Qinglongtan in S County. At that time, the witch of the Qinglongtan Water Palace suffered the "wind disaster" among the three disasters, and the old man learned the news and sent someone to Qinglongtan to collect this rare eagle wind for later experiments. Although the wind was so powerful that it almost killed the witch, the old man used the principle of aerodynamics to make a flying machine, subdued it, and used it for his own use.

The assistant released the wind from the bag, and the powerful wind flew to the front baffle like a blade, and the wind flew through the baffle in front of it, cutting knife marks on the screen. The blade-like wind slashed at the screen, eventually forming two parallel grooves corresponding to the slits on the screen.