31st, shopped at Koubundou in front of the station and knows something".

X was buying something. That has already been established.

At the same time, he was doing something conspicuous and was also committing a crime.

Koubundou carries equipment for elementary school students, so they probably don't have anything valuable.

That reminds me, the newspapers were crammed with disturbing reports of crimes, as usual. There was arson, robbery, murder, and...

I sighed deeply.

"Seriously..."

"Seriously?"

One high school student entered the shop that sells stationery to elementary school students in his free time. He placed a cheap item on the counter somewhat hesitatingly and took out a 10,000-yen note, thereby drawing attention to himself.

"X used a counterfeit 10,000-yen note to pay for his goods."

"But!"

Chitanda, who had kept silent and had not stirred an inch after my words, suddenly piped up. She then started talking on and on, her spell of silence having been broken with that one word.

"But but but that's impossible! It's unrealistic! It's illogical! This is a failure! A catastrophe!"

Seeing Chitanda's extreme vigor and sensing that she might just leap out of her seat and strangle me, I drew back my chair and retreated.

I stopped Chitanda with a hand gesture, while thinking that that was probably similar to calming down a violent bull.

"Ch-chitanda, please calm down. Ah that's right! I just remembered! This is just a game, right? You won't get anything out of it by being so serious."

"But it's impossible!"

Hmm. She used the word "impossible", instead of "unbelievable"?

I narrowed my eyes and asked Chitanda.

"What do you mean by impossible?"

Chitanda had both her hands on the table and was leaning over it, but she returned to her original position and coughed slightly, as if embarrassed by her own actions. She put on her usual expression and spoke.

"The counterfeit bills that have been used are 10,000-yen bills. You concluded that X-san used a 10,000-yen bill based on that news, right?"

I nodded.

"But there is no way X-san, a high school student, could have gotten their hands on one! Or even if they did, they should have been able to exchange it!"

"What do you mean?"

That was actually quite slow of me. I have no idea why I even asked Chitanda that question. Chitanda's face showed a flicker of irritation, but she continued.

"How did X-san, a high school student who does not sell anything, get hold of the counterfeit bill?"

I answered without giving it much thought.

"From an ATM, I suppose."

"It's no easy feat to forge bills good enough to trick an ATM or a bank! And if they were that good, it would be unusual for X-san to notice."

"Perhaps he received it as change?"

After saying that, I shut my mouth, taken aback by my own stupidity. I was fortunate that Ibara wasn't here. Who knows what mean things she would say about me? Since Chitanda was not Ibara, she looked at me and smiled instead of criticizing me.

"Exactly. It seems that you have realized. A 10,000-yen bill cannot be given as change. Apart from commemorative coin, the 10,000-yen bill is the largest bill in our country."

I finally understood why I asked Chitanda that question.

How did X get hold of that fake currency for him to be able to use it? A counterfeit bill is one made by an unlicensed source, and is made to be used in shops. When it does, it shouldn't be handed over to customers, and even if it's passed in between shops, it would eventually go to the bank. That's where it stops.

With furrowed eyebrows, I nodded by a few degrees.

"Well, I get what you're trying to say. Even if X's father ran a business, received a fake bill and gave it to X as pocket money..."

Satisfied, Chitanda nodded.

"X-san would have told their father. They would have asked their father to exchange the bill."

Kamiyama High prohibits her students from working part-time, but even if X did do it, that doesn't change anything. If X received their payment by bank transfer, there is no possibility of them even touching the fake note. On the other hand, if X received the money physically, they should have been able to request for an exchange, given that the employer is not overly unjust. Just like for the possibility of Student Counseling Department collectively collapsing due to food poisoning, I did not really consider the case for X's employer or father being so cruel as to reject that request, because that would be too nonsensical.

Now, then...

"Perhaps they picked it up?"

"Picked it up? You mean that the fake note was left on the road?"

"The forgers might have disposed of it to get rid of the evidence, for example."

It was quite an absurd theory, but only in its basis, and I didn't really care too much.

However, Chitanda shook her head.

"That would also be unlikely."

I was about to ask why, when I realized something strange.

Assuming that X went to school on both days, he would have delivered the note to Koubundou sometime between the end of yesterday's school day and the start of school today. Even if he didn't, the time frame for the delivery of the note would be from yesterday morning to today's announcement. The time taken for both cases is way too short.

X felt guilty while he was using the counterfeit bill. If not, he probably wouldn't have regretted his crime and apologized so quickly. It certainly wouldn't be possible for someone who uses a fake note he just picked up on an elderly couple to earn some small change.

"Hmm. The method of obtaining the bill, huh..."

If you don't figure that out, your whole theory would be like a tower built on sand."

Hey, you mentioned that I say rarely used phrases, but aren't you just the same?"

Even as I laugh it off, I have to appreciate Chitanda's focus. It might be a small thing, but as they say, a little leak will sink a great ship. How did X get hold of the fake note, and why did he have to use it?

Or is the theory that I've built up completely wrong, as Chitanda is saying?

A murmur escaped from my lips.

"10,000 yen, huh."

That's definitely not that large a sum of money that people would dream about it. But then again, it's also an amount that one would regret losing[4].

... I see. It's an amount that one would give up only with painful reluctance. I folded my arms.

"Chitanda, do you like money?"

Chitanda was a little flustered by the question, but she still answered.

"Yes, I suppose. I'll have to say that I like it rather than hate it."

"Would you be troubled over throwing away 10,000 yen?"

"I don't think so."

As if this was an important matter, Chitanda drew closer and added solemnly,

"... But only if it were not from an illegitimate source."

You sure are protected, Chitanda. People have killed for less, even in Japan.

But even as I thought this, I could understand what Chitanda was saying. If the 10,000 yen was rightfully mine, I definitely wouldn't want to lose it. If I did accidentally misplace it, I would even clean out the drains in the city in an attempt to find it. But if it were "illegitimate money", or in other words if it was money that was picked up, stolen, or earned from gambling, I would probably give it up as easy money. As they say, "Ill-gotten gains are soon spent."

That can be the only reason why X felt guilty and yet still used the counterfeit bill. He did not want to waste his own money. In that case, that 10,000 yen was not from an illegitimate source. Also, X is not a forger, or the accomplice of one. If that's the case...

"As I thought, X received the bill from someone."

Chitanda, who was staring at the notebook, looked up.

"And they received it as rightful money. If they did not get it as a salary or allowance, then there's only one possibility... It was given back to them in return for money they had previously lent.

"I'm sure X felt troubled after realizing that the money received was fake. They probably thought something like 'It was my money so why did this happen?' In any case, you can't really blame them for feeling guilty while wanting to use it at a shop run by an elderly couple."

As I finished, Chitanda placed her fist near her mouth and thought for a while. She eventually put down her fist and nodded, but she suddenly shook her head as she thought of something.

"But that doesn't change anything! X-san should still have been able to exchange the fake note!"

I replied calmly.

"Really? The counterfeit bill is like the Joker in Old Maid. No one wants to be holding it. In that case, I'd say this situation fits:

"'Yo X, here's the money I borrowed from you the other day.'

"'Ah, Y-senpai? Thank you very much. You could have returned the money later, though.'

"'It was 10,000 yen, right? Here.'

"'Thank you.'

"But to their surprise it turned out to be fake."

Chitanda didn't even grin at my solo act. Feeling awkward, I continued.

"Y, whom X lent the money to, was someone higher in social standing. That's why they couldn't object when Y gave them back a counterfeit. X could have noticed right after receiving the money, but Y could easily feign ignorance. Thus X got hold of the fake note under these circumstances."

I crossed my legs and continued.

"There was still the question of whether X is a single person or a group of people, but based on all this, we can say that X is only one person. It would be far too unnatural for two or three high schoolers to buy cheap stationery with a 10,000-yen bill together."

Chitanda still remained silent, making me wonder if she was actually listening.

There's still one more point that should be examined, I thought as I spoke.

"... So what about Y?"

"Y got hold of the counterfeit bill. Perhaps he got it from a Z higher up on the social ladder. But at some point it would have to return to the forger, a shop or the bank, as intended. We can group everyone above Y into Y as well, and then we have the question: Who is Y? An unscrupulous shopkeeper? The forgers themselves?

"In the messy trail caused by the counterfeit notes, the only way out was to turn to a lone high schooler who fell to a sudden impulse. The police got involved with X in order to trace where the bill came from."

I sighed deeply and shrugged jokingly.

"That concludes my deduction."

I noticed that Chitanda was deeply seated in her chair, which was quite strange. Her palms were resting on her knees, her back was straightened, and she had a blank expression on her face. She was probably shocked by the conclusion, or tired out from the game.

But even so, I spent all that time and effort talking, and she doesn't say a word? How mean. I left Chitanda to her sullen silence and gazed out the window at Kamiyama City, where the color of autumn had begun to show. Since Kamiyama Station is somewhere over there, Koubundou should be in that area too, right?

Chitanda's muttering reached my eyes as I stood there.

"'Anyone who, on October 31st, shopped at Koubundou in front of the station and who knows something, please report to Shibazaki in the Staff Room immediately.'"

She then spoke earnestly as I turned around.

"We've certainly come a long way from this."

"Yeah..."

I smiled and stretched.

"It's game over, then."

Chitanda raised her eyebrows at the word "game". Focus returned to her eyes.

Chitanda's head tilted slightly.

"Oreki-san."

"What? This is just a game, you don't need to take it seriously."

"No, that's not it, but if that was a game, I have the feeling that you started it to prove something... What was it again?"

Ah.

Now that I think of it, I did want to do something like that.

I also tilted my head, to about the same angle as Chitanda's. After school, at the Geography Lecture Room, two people shrugged together.

"What was it?"

"I wonder."

"It you can't remember, there's no reason I would."

"... Then how about we try to deduce what that was?"

Chitanda's lips had slackened, and while her large eyes were equipped with seriousness, I could see that she was smiling. Ah, whatever. I smiled the largest smile I could manage and spoke.

"Oh, give me a break."

The next day.

As I flipped to the third page of the newspapers, this article caught my eye.

"Caught for Possession of Counterfeit Currency"

The subheading read,

"23-year old gang member first to be arrested in a series of incidents, Kamiyama Police reports."

I believe we started that game yesterday because of some maxim. Both Chitanda and I completely forgot about it as we were so caught up with the game, but now I remember what is was.

That was "A horse comes from a gourd"[5], right?

... Yeah, I think so.

Well, for memory to match the truth, one will need a lot more than pure luck, I suppose.