This happened back when I was still in middle school. At the time, urban legends and ghost stories were all the rage in my class.

A few of my classmates—usually about six of us—would stay behind after school, gathering in the classroom to swap stories like that.

There was one girl, A-chan, who was an especially good storyteller. Even if it was a cliché horror story with a predictable punchline everyone had heard before, she’d tell it in a way that gave you goosebumps. I was secretly a big fan of hers.

One of the stories A-chan told us was about "how to spot someone who doesn't belong to this world." After hearing it from her, I ended up becoming able to spot them too, which is why I decided to share this story today.

This is an experience that A-chan and I actually went through.

First, let me tell you how A-chan became able to spot them.

On the route A-chan took to school every day, there was a fairly large shopping mall. It was a bustling area with constant foot traffic and crowds of people coming and going.

A-chan began noticing "a certain person" there.

In the middle of a busy intersection, there was someone just standing frozen on the sidewalk—not attempting to cross the street at all, just standing stock-still.

Since A-chan didn't cross at that crosswalk and just walked straight home past it, she said she could only ever see that person's profile.

About a month after she first noticed them, one day that person was standing at a completely different angle.

Their back was turned to the crosswalk, facing directly toward the sidewalk where A-chan was walking.

Realizing it was the first time she would actually see their face, A-chan casually glanced over at them.

When she did, she saw that the person’s mouth was hanging wide open, gaping vacant. And A-chan ended up looking right inside that person's mouth.

She hurriedly looked away, but the image of the inside of that mouth refused to leave her head.

The moment she saw inside their mouth, she said she instinctively knew: "That's not a living human."

When I asked her why looking in their mouth made her think that, A-chan just smiled and said, "I'll tell you next time," refusing to explain then and there.

I kept waiting for that "next time," but not long after that, A-chan stopped joining our after-school gatherings.

In fact, she stopped talking to our classmates almost entirely.

I was worried about why she was suddenly acting like that, so during lunch break I went to ask her about it. I asked her why she hadn't been staying after school lately.

That was when A-chan replied to my question with: "It's red, right?"

Her answer was so completely disconnected from my question that for a second I couldn't even process what language she was speaking.

It probably only lasted a few seconds, but in my head, I slowly pieced together the words she had just uttered.

I t ' s r e d , r i g h t ?

Even after stringing those words together in my head, I still couldn't understand what she meant.

Besides, the way A-chan spoke was so cold and devoid of emotion—so utterly terrifying—that I couldn't bring myself to ask her what she meant.

When I told a close friend about it, they just laughed and said she was probably making fun of me, but it didn't feel like that at all. Still, I lost the courage to speak to A-chan again, and decided to just forget about it.

But despite wanting to forget... it happened after school that very same day.

By the shoe lockers, A-chan approached me to talk this time. My heart skipped a beat, but A-chan looked completely normal then, and even apologized, saying, "Sorry for asking that weird question earlier."

But as we kept talking, her behavior gradually started to get strange again.

She kept wringing and rubbing her hands together nervously, while her eyes remained glued to a spot on my face. Her restless, anxious fidgeting and unblinking gaze made me feel increasingly uncomfortable, and just as I was about to tell her I had to head home...

A-chan suddenly blurted out to me, "I was looking inside your mouth."

I asked her what she meant, but A-chan didn't answer. A creeping dread washed over me, and I turned and ran home to escape her.

Even looking back now, I can't remember what I said to get away from A-chan and run home.

I don't remember anything after getting home, nor do I have any memory of how I spent the following days at school sharing a classroom with A-chan, but a short while later, she transferred schools.

It wasn't a normal transfer because her parents moved; she was transferred to a hospital-based school classroom.

All sorts of rumors flew around the class—that she'd been in a car accident, or that she was suffering from a severe illness and couldn't attend school anymore.

A few students asked the teacher for her contact information because they wanted to see her, but the teacher refused to give it out. All he said was that she was in no condition to receive visitors.

Everyone loved gossip, so for a while people amused themselves by embellishing the reasons for A-chan's transfer with all kinds of wild theories, but eventually, the subject faded away.

Even after people stopped talking about it, what A-chan had said about "inside the mouth" remained stuck in my mind.

I'm certain the person standing by the shopping mall crosswalk that A-chan saw wasn't someone belonging to this world.

And I couldn't help but think that whatever A-chan saw in them was the reason she ended up hospitalized.

Still, I didn't have the courage to go check out the shopping mall myself, nor did I dare try to get in touch with A-chan.

I just wanted to forget everything.

I was absolutely terrified by the intense, boring stare directed at me after school that day, and by the sound and sight of her rubbing her hands together over and over.

For about a year after A-chan left, I went through my days pretending to have forgotten all about it.

It was about a year later, I think. During a conversation with some friends, someone suggested we go shopping at the mall after school.

Honestly, I didn't want to go.

However, that mall was pretty much the only one in the town where I lived, and going to the neighboring town was difficult without a car.

Reluctantly, I agreed and headed to the mall after school.

The shopping mall had two main entrances, and both had large crosswalks in front of them. I had no idea which crosswalk A-chan had been talking about, but the instant I saw it, I just knew.

Even from a distance, I knew: That's the person.

Sure enough, there was a person standing in front of the crosswalk. But just like on the day A-chan had looked inside their mouth, they were standing with their back turned to the crosswalk.

Alarm bells were screaming in my head: "Do NOT look at that person!"

I should have been hearing the noisy bustle of pedestrians and passing cars, but all of that suddenly fell dead silent, replaced by the wild pounding of my own heart acting as an alarm.

And yet, why? As if drawn in by an irresistible force, my eyes drifted toward that person anyway.

A mysterious pull that couldn't be explained merely by morbid curiosity took hold of me, riveting my gaze onto them.

As I got close enough to discern their facial expression with the naked eye, I saw it too. A gaping, wide-open mouth.

The inside of the mouth that had bothered me for so long.

A-chan's way of spotting people who don't belong to this world.

The person standing there was definitely not a living human being.

They had the shape and appearance of a human, but they were unequivocally not one. The secret was indeed inside their mouth.

Inside that person's mouth, there was nothing at all.

Truly, nothing.

The inside of their open mouth was pitch black, but not the kind of black painted over with ink or dye.

There was supposed to be a throat and a tongue, yet you couldn't see anything. An endless, bottomless abyss of black stretched on forever inside that person's mouth.

It felt as though if our eyes met, I would be sucked right into it.

Snapping back to my senses, I squeezed my eyes shut tight and hurried past the spot.

The moment I passed them, the surrounding noise of the crowd suddenly rushed back into my ears.

Oblivious to everything, my friends were chatting happily beside me, and hearing their voices brought me back to reality for the first time. I remember feeling profoundly relieved.

When I opened my tightly shut eyes, the black void was gone from in front of me, and the usual scenery lay spread out before me. But almost immediately, I realized: "Ah... it's not over yet."

That whole day, I couldn't turn around to look behind me.

They were there. Right behind me, the whole time. The sound of hands rubbing together kept echoing in my ears.

I didn't end up having a nervous breakdown after that, and now I've grown into an adult, but even now, I still occasionally remember the inside of that mouth I saw that day.

And ever since that day, I've occasionally been able to see people who don't belong to this world.

Even in my daily life, I spot them every now and then.

On the corners of streets, in public restrooms... literally everywhere. People standing with their mouths gaping open.

I think A-chan was swallowed up by that pitch-black mouth.

I think she was drawn into it, stepping closer and closer to that person.

And surely, at the end of that deep black mouth...

"It's red, right?"

I still don't know whether those words were meant for my mouth or for A-chan's, but to this day I regret that if only I had told her, "Yes, it's red," maybe things might have turned out differently.

I'll never know for the rest of my life where A-chan is or what she's doing, but she might no longer be in this world.

I can't help but hope she's doing well somewhere, but a part of me suspects that "that sound" belonged to her.

And from the bottom of my heart, I am so glad I managed to close my eyes back then.

Because if I had been sucked into that blackness as well, I'm certain I would have stepped out into the crosswalk and died.