After asking his question, Wang Shen looked at Liang Zichen. Liang Zichen glanced at Wang Shen and nodded.

"Moreover, switching the keys is not only difficult but also risky," Wang Shen said, pursing his lips and staring at cabinet number 16.

"Captain Wang, does the risk you're referring to mean that the murderer already guessed we'd use this key to open cabinet number 16?" Liang Zichen immediately understood what Wang Shen meant.

“That’s right. Look at the arrangement of the cabinets now. It’s obvious that the murderer restored the order of all the cabinets to its original state. The purpose of doing so is obvious: to eliminate any remaining potential hidden dangers we might have discovered through the substitution. But the murderer didn’t expect that I would accidentally take a picture of this entire cabinet wall on the day of the incident, thus uncovering his trick. However, after restoring the order of the cabinets, if we can’t switch the key back with A-Cheng, then we’re now holding the key to cabinet number 17. If that’s the case, we won’t be able to open cabinet number 16 no matter what. In that case, the murderer’s plan will be completely ruined in an instant. And to switch the key in such a short time is too difficult and too risky.” Wang Shen stated his deduction in one breath.

Liang Zichen listened to Wang Shen's explanation while nodding repeatedly. "Since switching the keys was too risky, how did the murderer manage to let us open cabinet number 16 so easily?"

"Maybe the key wasn't switched at all, and what we have is actually the key to cabinet number 17?" Wang Shen said, looking at the key and the cabinet door.

“We have the key to cabinet number 17, but we’re opening cabinet number 16.” Liang Zichen was confused by Wang Shen’s words again. He couldn’t understand how the key to number 17 could open cabinet number 16.

Wang Shen did not answer Liang Zichen's question immediately, but instead reopened the cabinet door of number 16. He then peered closely at the lock cylinder inside, examining it carefully for a long time.

Suddenly, a smug smile appeared on Wang Shen's face. "That's the problem," Wang Shen said, pointing to the lock cylinder on the cabinet.

Upon hearing this, Liang Zichen immediately brought his eyes closer to the lock cylinder, examining it carefully. After staring at the lock cylinder for a while, Liang Zichen frowned slightly: "Captain Wang, there's something wrong with this lock cylinder."

"Yes, you noticed it too?" Wang Shen replied with a smile.

“That’s right.” Liang Zichen nodded. “This cabinet looks like it hasn’t been cleaned in a long time, so there’s a lot of dust on the doors. But it’s strange, the dust around the lock cylinder seems to have been cleaned irregularly.”

Wang Shen listened and nodded. "You're quite observant, but the dust around the lock cylinder probably wasn't cleaned intentionally; it was just accidentally rubbed off."

“Yes, I think so too. Not only that, Captain Wang, look here.” Liang Zichen pointed to a screw on the lock cylinder. “This screw doesn’t seem to be exactly the same as the other screws on this lock.”

"That's the biggest flaw. So can you now guess how the murderer made us use the key to cabinet number 17 to open cabinet number 16?"

Liang Zichen thought for a moment, then nodded and said, "I suspect the original screw was accidentally left behind by the murderer during the lock replacement process. So in their haste, they found this substitute. The murderer's method of substitution was to ensure the cabinets were in the correct order before changing the locks. This way, we can use key number 17 to open cabinet door number 16. How clever!"

“That’s right, it all makes perfect sense. And, just as the murderer predicted, on the day of the incident, without this key, we wouldn’t have been able to uncover his trick.” Wang Shen’s smile faded, replaced by a solemn expression. “Therefore, the act of changing the lock cylinder must have happened after we left the store. And I suspect the murderer is someone from this store.”

"Captain Wang, are you referring to Han Lin as 'the person in this shop'?" Liang Zichen looked at Wang Shen, though he himself had the same guess.

"To be honest, I think so. But such a guess is a bit arbitrary, since we don't have any concrete evidence. So our next task is to collect evidence from various sources." Wang Shen nodded, then shook his head slightly.

"Captain Wang, wait a minute. I've discovered some other problems in the design of this trick." Liang Zichen looked at the cabinets, suddenly realizing something, and said to Wang Shen with a serious expression. "After the cabinets are switched, A-Cheng needs to open or close his own cabinet door, which requires key number 17. But initially, key number 16 was definitely next to A-Cheng. Simply put, the murderer had to switch A-Cheng's key number 16 with key number 17 before having A-Cheng use key number 17 to open cabinet number 17 and take the prepared clothes and gloves. How did the murderer manage to switch key number 16 next to A-Cheng?"

"I've already thought of the answer to your question. The murderer is a rare genius," Wang Shen replied, frowning slightly as he looked at the cabinet in front of him. "First of all, the murderer must have been very familiar with A-Cheng's lifestyle." After saying this, Wang Shen walked towards the small door inside the dressing room. He gently pushed the door open and went inside.

"Why is there a bathroom here?" Liang Zichen asked in surprise as he followed behind Wang Shen and looked at the small bathroom in front of him.

"This is a small bathroom provided for the employees, for their convenience," Wang Shen explained simply. "In addition, I asked Han Lin about Acheng's work habits. Acheng always comes to the store very early every day because he has a habit of taking a shower in the morning, probably to avoid other peak hours."

"But what does this have to do with the key swapping?" Liang Zichen was a little anxious and seemed to have no interest in listening to the story about the bathroom.

“Okay, then let’s talk about the key swap. Tell me, when could the key swap happen?” Wang Shen turned to look at Liang Zichen.

"When were the keys switched? I can't guess." Liang Zichen shook his head at Wang Shen, then seemed to have figured something out. "No, to avoid any loopholes, the shorter the time of the key switch, the better." Liang Zichen replied, then paused for a moment, before retracting his previous deduction. "Still not right. Acheng needed to open the cabinet to get something, and clothes splashed with gasoline can only be used once, so the key switch must have happened within 24 hours before the incident."

“That’s right!” Wang Shen gave Liang Zichen a thumbs up. “But to switch the keys in such a short time, we need to find a good opportunity to do it. When is that good opportunity? It’s during the time when A-Cheng comes to take a shower this morning.” Wang Shen said with a frown.