We all put on gloves. In one corner of the room, an outline of a body had been chalked onto the floor—this was likely where the victim had lain. The room was cluttered with various chemical substances, and I moved toward the table in the southeast corner. It was his grandfather's. The table was filled with documents, pens, pencils, an electric microscope, and more.
Wait, what's this? It looks like there's a hidden compartment inside a drawer. It's locked. There's also a peculiar pattern here. It's clearly designed for a key. The pattern has been carefully etched into a thin layer of plastic, something that reminds me of those templates used for fabric designs. But on closer inspection, I realized the pattern forms a blueprint of the entire room. It's telling me that I'll find the key somewhere in the north, right beneath the air conditioner.
As I moved towards the AC, Hasan suddenly spoke up, "On that day, the room temperature was set to fifteen degrees Celsius."
Fifteen degrees? Isn't that way too cold?
I pushed aside the mat and found the key beneath it. When I turned around with the key in hand, I saw that Rima and Hasan were taking a break, clearly exhausted but surprisingly not unhappy. They seemed to be enjoying this for the first time despite the hard work involved. Just then, Meem called out, her voice brimming with excitement. Hearing her, I felt a spark of hope. She really was the responsible one—always engaged and active in everything—opposite to me.
We all gathered around her. "Did you find something?" I asked.
Meem nodded and pointed, "There's some white powder beneath the mat by the door."
"Where?" I asked, crouching down. "Let me see."
Indeed, there was a faint dusting of white powder. I picked it up and examined it closely.
"It would be good to know the properties of this powder," I mused aloud.
Meem chimed in, "If there are enough chemicals here, I might be able to identify it."
Hasan gestured towards the shelves. "There are plenty of chemicals to work with."
"Alright," Meem said. "Rima and I will run a test."
They got to work, running tests while Hasan continued searching the room. After a while, he called out, "Sakib, look what I found under the table—a tape recorder."
I took it from him. The timer was set. According to the forensic report, the recorder had been set about a minute or two after his grandfather's death. I pressed play, and a familiar voice came through the speaker. Hasan confirmed it—it was indeed his grandfather's voice.
At the same time, Meem and Rima finished their experiment. They announced, "It's calcium chloride."
Salt, a low temperature, and the tape recorder... These clues pointed towards one thing.
With a slight laugh, I muttered, "What a fool."
Meem stared at me in confusion. "Why are you laughing like a madman?"
Still chuckling, I replied, "I couldn't catch the culprit. But I have solved the locked room mystery."
They all looked at me, stunned. "Really?" they asked in unison.
"Yeah," I said with a casual wave of my hand, "it's a pretty simple trick. Even a ninth or tenth grader could pull this off."
Their eyes narrowed in disbelief. "What?" Hasan exclaimed.
Meem said impatiently, "Stop being cryptic and just explain it already!"
I turned to them and posed a question. "Do you know how two pieces of ice can be fused?"
Meem answered right away, "When pressure is applied to two ice cubes, the melting point at the point of contact decreases. The ice starts melting at that spot, and once the pressure is released, the water refreezes, fusing the two pieces."
"Exactly," I said, nodding. "There was salt-infused ice in the metal lock of the door. As Meem explained, the ice fused, creating the illusion that the door had been locked from the inside."
Hasan scratched his head, still puzzled. "But why use salt in the first place? Wouldn't plain ice work just as well?"
I threw another question back at them, "What's the melting point of water?"
"Zero degrees Celsius," Hasan and Rima answered in unison.
Meem corrected them, "That's not the melting point of water; it's the melting point of ice."
"Oh, right," Hasan admitted, embarrassed by his mistake.
I then asked Hasan, "What did you say the room temperature was?"
"Fifteen degrees," he confirmed.
"They used salt to raise the melting point of the ice," I explained.
Rima wasn't convinced. "Hold on. I don't get it."
Meem agreed. "Yeah, me neither. I've always known salt is used to lower the melting point of ice, like how it's used to melt snow in colder countries."
I glanced at Hasan. "Can you set the room temperature to twenty degrees?"
He grabbed the remote and adjusted the air conditioner. After waiting for a few minutes, I had him bring me a glass of cold water. Then I retrieved the container of calcium chloride from the shelf and poured it into the glass. Slowly, the water began to freeze.
Hasan and the others watched in amazement. "Look at this. At twenty degrees Celsius, I just turned water into ice using salt."
They were silent, clearly impressed by the demonstration.
"This," I explained, "is calcium chloride. It has a melting point close to twenty-five degrees Celsius. Its crystalline structure is quite unique. The perpetrator used it in the lock to fake a suicide. When the victim was murdered, the lock was sealed with salt-infused ice, creating the illusion that the door had been locked from the inside."
"So, who applied the pressure to fuse the ice?" Rima asked.
"Take a look at the lock mechanism. It's made of magnetic metal. There's a strong magnet in the lock's chamber. Once the door was shut, the magnetic pull applied enough pressure on the ice to fuse it. And the low AC temperature was used to keep the ice from melting too quickly."
"But can ice really become that strong?" Hasan wondered.
"Be a little patient. Give me some water." I was getting tired of answering all these questions. My throat was parched. After drinking some water, I said, "Okay, now ask your questions."
"Can ice really get that strong?" Hasan asked again.
"Yes, of course. Newly formed ice is stronger than old one." Hasan scratched his head in confusion. "What are you saying? It's all going over my head."
"Look, what's the relationship between mass and strength?"
"Mass and strength are directly proportional, meaning if the mass increases, the strength will also increase," Meem answered.
"Exactly. The mass increased here, so naturally, the strength increased too."
"But can magnets interact with ice?" Hasan asked again.
"They can. You see, most magnets aren't affected by cold temperatures. In fact, some perform better when it's cold. The reason is that magnetic molecules vibrate slower and more randomly when cooled. This results in better alignment of the atoms that create the magnetic field, increasing its strength."
Meem was satisfied with the answer. "Oh, so that's the story behind the salt."
"I get it. But why did they use the ice theory? The door could have locked automatically," Rima asked after thinking for a while.
"Good question. Have you noticed one thing? Hasan's grandfather was almost seventy years old, but he used to come to work on the fifth floor of a building. Can you tell me why?"
"Yes, Grandpa used to exercise to stay fit. He would go jogging in the morning and handle all his personal work," Hasan said.
"Now do you understand? Anyone knocking on the lab room door would have had the door opened by him. It was part of his fitness routine. The killer must have been someone close to him who knew about this habit. So, to create the illusion of a locked room, the killer used the ice theory. When they pushed to open the door, some of the ice broke and fell. This ice melted into water and eventually evaporated, leaving only the salt behind," I explained to Rima.
"Then why did they use a tape recorder?" Meem asked again.
"The tape recorder was set to play right after Hasan's grandfather was killed. I think the moment the killer was sure the ice had sealed the lock, the tape recorder started playing. At that point, everyone broke through the door and entered," I replied.
This time, they agreed with me. "Your explanation makes sense," Hasan said.
"So, the locked room mystery is solved," Meem said.
"Yes, but now we have to catch the killer."
Hasan said, "The killer took the laptop. Once we catch the killer, we'll get the laptop too."
I smiled slightly and said, "We don't need to catch the killer to find the laptop. The laptop is still here."
Everyone was surprised by my statement. Hasan asked, "How can you be so sure?"
"According to the post-mortem report, your grandfather died twenty minutes before everyone arrived. It would take the killer at least fifteen minutes to lock the door using the ice and set up the tape recorder. That leaves only five minutes. At that time, it's unlikely the killer could have hidden the laptop far away. If they tried to leave the crime scene in those five minutes, they wouldn't have made it back in time, raising suspicion. So, the killer must have hidden the laptop right around here," I explained.
Hasan accepted my reasoning. "Okay! But finding the laptop in this large lab will be difficult."
I smiled again. "It's not difficult at all. In fact, it's an easy task."
Meem, surprised, asked, "How?"
"Look, the killer only had five minutes. During that time, they hid the laptop and then rejoined everyone at the crime scene. So, they only had one or two minutes at most to hide it. If my guess is correct, the laptop is hidden either on the right or left side of this room, or under or behind one of the tables in front of us. The three of you should search thoroughly."
They started searching. Meanwhile, I opened Hasan's grandfather's secret drawer. Inside were two diaries. In the first diary, there was a photo. I called Hasan over and asked, "Whose photo is this?"
Hasan came over. "This is my grandfather and grandmother's photo. Where did you find it?"
"In this diary."
After looking at the diary, Hasan said, "Oh, this diary. Grandpa traveled to many countries. He used to record his travel experiences in this diary."
"Oh, I see. But there's something I don't understand."
Hasan, puzzled, asked, "What?"
"According to the post-mortem report, you all heard your grandfather's voice twenty minutes after his death when you entered the room. But during the time of death—when he was killed—no one heard any voice. How is that possible?"
Hasan said, "Good question. I don't understand that either."
I stared at Hasan in astonishment and thought, it was a mistake to even ask this guy.
Suddenly, Rima called out to us. "The laptop is found."
We were all relieved by the news. "Well done," I congratulated her. "Now it's the killer's turn. The killer might come back for the laptop. Leave it where it was."
Meem asked, "Why?"
"Don't worry. We'll use the laptop to catch the killer."
They trusted me and put the laptop back. Meem said, "There's nothing else here that would be useful for the case."
"Okay." I said to Hasan, "Can you take me to your grandfather's private room? Let's see if we find anything important there."
"Alright, let's go."
The room was fairly large, enough for two people. On one side, there was a wooden bed. It was made for one person, without any decorative designs, which I really liked. In front of the bed, there was a table, and next to the table, a bookshelf full of scientific books. There was also a wardrobe near the door with only a couple of shirts hanging inside. I noticed a hole in the wardrobe door, probably made by wood-eating insects. On the right side of the bed, there was another bookshelf, and on the left, a washroom. The room also had a south-facing balcony. The door to the balcony had labels with names and uses of medicinal plants. This meant the plants on the balcony were medicinal. The plants were still fresh. I asked Hasan, "Who waters these plants?"
"These were my grandfather's favorite. Now I water them."
"Ah, I see. Thoroughly search the room to see if you find anything."
Everyone started searching as I instructed. I focused on the area next to the wardrobe, wondering if there were any hidden compartments. I found a basket beside the wardrobe filled with papers and medicine wrappers. I checked the papers, and they had some chemical reactions written on them. The organic reactions seemed eerie, almost like something from a ghostly realm. I put the papers aside and picked up the medicine wrappers. Some were for blood pressure, which my father also takes, but there was one I didn't recognize. I asked Hasan, "What is this medicine for?"
Hasan came over and examined it closely. Someone had tried to scratch off the name printed on the aluminum strip. "This is a sleeping pill. I used to dissolve it in water for my grandfather when he was really stressed."
"Sleeping pills!"
I took the strip back from Hasan and studied it carefully. "When was the last time you gave him this medicine, and how many pills were left in the packet?"
"I gave it to him two weeks ago, and there were three pills left."
As soon as Hasan said that, his eyes widened. That meant someone had given his grandfather the sleeping pills without his knowledge. I said, "It was mixed with the syrup-based drink in the glass on the table."
"Did you figure that out from the ants around the glass?" Rima asked.
I nodded. Everyone fell silent.
"The killer was very clever. The way they locked the door was ingenious, and they made very few mistakes." The silence in the room bothered me. So, I turned to Hasan and said, "Hasan, can you do something for me?"
Hasan looked at me with eager eyes. "What is it?"
"Tell your uncle to call the seven lab assistants for questioning. They won't come if we call them, but if you explain everything we've found, your uncle will become more interested in the case."
Following my suggestion, Hasan brought his uncle, Mr. Anisul Islam. He was a well-built man, quite stout, and about my height. When he removed his hat, his bald head became visible. He appeared to be around forty and was currently serving as an Assistant Sub-Inspector. He was pleased with the information we had uncovered. He asked me, "What's your name? And what's your relation to Hasan?"
"I'm Sakib. Hasan is my classmate; we study in the mathematics department."
"How did you figure all this out?"
With a smile, I replied, "You're mistaken if you think I did everything. They collected all the clues, and I just made deductions based on those hints. Anyway, I believe you can easily catch the killer now."
Mr. Islam laughed heartily. "Of course! You've solved seventy percent of the cases. If we can't handle the rest, we shouldn't even call ourselves police!"
He paused for a moment, then asked, "So, what do you think we should do now?"
"Call the seven lab assistants and question them thoroughly."
"Hmm, you're on the right track. That's exactly what I wanted to hear from you. Now it's my job to interrogate those seven."
"Yes."
He called the seven lab assistants, and when they arrived, he started questioning them in the sitting room. We kept an eye on them from a distance. I had a hunch that one of them was the killer. Apart from that, there was a person who regularly cleaned the lab, but the sophisticated method used here was beyond his capabilities. Many questions were asked about the events of that day, and they all answered very skillfully. Their responses were flawless as if the killer was confident. After the interrogation, Mr. Islam approached us and asked, "Did you figure anything out?"
I shook my head, indicating no.
"They gave me the same answers that day too. With their confidence, it doesn't seem like this is a murder case." He spoke with frustration.
After hearing this, we all fell silent. Hasan asked, "What should we do now?"
His tone showed his disappointment, which was understandable. After coming this far, no one would want to turn back. But there was still one way to catch the killer. I comforted Hasan, "Don't stress. There's a way."
Hasan eagerly asked, "What way?"
I couldn't tell him the plan right then because it was risky. So, I said, "Bring the laptop."
Rima brought the laptop as I had asked. I handed it to Hasan and said, "You'll stay in your grandfather's room tonight with the laptop."
Hasan agreed and left. I then told Mr. Islam, "The killer will come tonight to kill Hasan. You need to hide and keep guard. But be very careful not to alert the killer, or they'll become even more cautious."
Mr. Anisul Islam understood my idea and assured me he would do everything in his power to protect Hasan. Then I turned to Meem and Rima, saying, "There's one more thing left to do."
Rima asked, "What's that?"
Meem said, "I get it, we need to spread the news that the laptop has been found."
I smiled and said, "Exactly, you've got it."
We all went to the sitting room. The seven lab assistants were chatting with each other. Hasan's uncle went up to them and said, "Thank you all for your time. We have already found the laptop and handed it over to Hasan, Abdul Khalek's grandson. He will stay in the lab tonight. Tomorrow, we'll submit your experiments for FDA approval. I hope your hard work won't go in vain. You may leave now."
After they left, Rima came over to Meem and me and whispered, "Did we just alert the killers?"
Meem replied, "You're wrong. This was a psychological attack on the killer, which was Sakib's plan. Now the killer will be thinking about how to retrieve the laptop tonight. If Hasan submits the research tomorrow, all their efforts will be wasted."
I was pleased with Meem's explanation. She had presented the plan perfectly. Hasan's uncle placed his hand on my shoulder and said, "Yes, Sakib's plan is great. Now, we just have to wait till night and stay vigilant."
Do we have to wait? Until night? No, we can't waste the night like this. "I think our work is done. We'll go home…"
As soon as I mentioned going home, I saw Meem's expression and got scared. Then I quickly added, "One way or another, we'll catch the culprit tonight."
There was nothing else to do. I took out my phone and started reading manga.