Towards the end, the young policeman's words became somewhat disjointed. He paused, then suddenly had a flash of insight. "So, the deceased's left-handedness must have been present since childhood. A habit maintained for decades becomes internalized into a person's daily behavior. Even under special circumstances, if one doesn't consciously change it, the original habit will remain. And I can't think of any reason why the deceased would deliberately change their habit and place the water glass on their right. Based on this, I deduce that the last person to handle this glass was not the deceased, but the murderer." After stating his deduction, the young policeman pursed his lips and nodded to himself. Seeing that Wang Shen and everyone else present were watching him intently, the pressure in his heart eased slightly.

“Go on, if you are the murderer, what can we deduce?” Wang Shen nodded to the young policeman, signaling him to continue.

“If the person washing the cup was the murderer, then their purpose was likely to prevent us from discovering any remaining clues on the cup. What exactly are these remaining clues? Based on the clues we have so far, it's very likely a drug used to incapacitate the victim. As for why they refilled the cup halfway with water, the murderer probably wanted to create the illusion that the victim was drinking water normally. These are my deductions.” The young policeman nodded slightly to Wang Shen, and only after seeing Wang Shen smile did the young policeman feel relieved and sit back down in his chair.

“Xiao Liang’s speculation just now is basically the same as what I was thinking. But Xiao Liang just raised another question.” Wang Shen also paused and glanced at everyone. “What exactly was the liquid that was originally in the cup? Was it mixed with a drug that would render the deceased unable to resist?” As Wang Shen spoke, he turned his body to the projection screen again and gently tapped the screen with his finger.

The image on the projection screen changed again. A carpet appeared in the picture, and a water stain was faintly visible on it. "Everyone can take a look at this carpet. To be precise, you should look at the water stain on this carpet. I spoke with the deceased's secretary about this water stain. The secretary admitted that the water stain was related to her, explaining that the deceased had a habit of drinking coffee, and when she went to deliver coffee in the morning, she discovered the deceased's body and, in her panic, accidentally spilled the coffee on the floor."

“It seems this secretary is quite problematic,” another slightly older police officer immediately commented after Wang Shen finished explaining the facts he had learned.

"Old Li, what's your opinion?" Wang Shen looked at the policeman who had spoken, and smiled as he gestured for him to continue his deduction.

The older police officer coughed, took a sip of tea, paused for a moment, and then said, “You mentioned earlier that the deceased was left-handed. His secretary couldn't possibly have been unaware of this. Therefore, the secretary would have placed the coffee on the deceased's left side whenever she brought it. Now, let me ask everyone to observe the location of this water stain. Clearly, the water stain is on the right side of the office chair. This presents a contradiction. The first contradiction is that the secretary violated her usual coffee-delivery habits. The second contradiction is the reason for spilling the coffee. The secretary said she spilled the coffee because she was startled upon seeing the deceased. I was also at the crime scene today, and if I remember correctly, the left side of the desk faces the door to the consultation room. Therefore, the secretary had no reason to walk around to the right side of the office chair to find the deceased and then panic. This logic simply doesn't make sense. The normal logic is that the secretary entered the office, walked to the left side of the desk as usual, found the deceased, and in her panic, spilled the coffee cup, leaving the coffee stain on the carpet on the left side.”

“Yes, our police force is truly full of hidden talents. Everyone is incredibly capable.” Wang Shen walked to the back of the conference table. “I had the forensic department analyze this coffee stain, and they came to very important conclusions. First, the stain did indeed contain traces of a date rape drug. Second, there was another major discovery. In addition to caffeine and other essential components of coffee, as well as the date rape drug, a large amount of soap base was also detected in the stain. To put it simply, this coffee stain was manipulated. Why manipulate this stain? The purpose should be obvious: to cover up or eliminate any remaining traces of the date rape drug. But the killer’s action was completely unnecessary.”

Wang Shen walked around the other side of the conference table and returned to the projection screen. "Based on the information we have, the deceased likely lost his mobility after drinking this cup of coffee. As for how the coffee ended up on the floor, there are many possibilities. For example, the deceased might have discovered the killer's intentions before he completely lost his mobility. At that moment, the killer happened to be standing to the deceased's right, so the deceased used his last bit of strength to throw the coffee cup at the killer, causing this coffee stain on the carpet. Let's leave that aside for now and discuss the relationship between the killer and the deceased. At least, I think the killer and the deceased were acquainted. Otherwise, the deceased wouldn't have felt comfortable drinking the coffee the killer brought. By saying this, it seems I've already identified the secretary as the killer. This is somewhat conjecture. So, I'll tentatively consider the secretary a suspect."

"But what about the motive for the murder? Between a man and a woman... could it be an emotional entanglement?" The older policeman pinched his chin with one hand, frowning as he looked at the coffee stain on the projection screen.

"Old Li is indeed very experienced." Wang Shen gave the older policeman a thumbs up. "Based on the clues I have, that's indeed the case." Wang Shen tapped the projection screen again, and the images on the screen changed once more. Two images appeared on the screen, each showing a cup. Everyone could immediately see that the two cups in the images were a pair of couple's cups.

“This cup,” Wang Shen pointed to the picture on the left with his laser pointer. “I took this picture from the deceased’s secretary’s desk. The secretary must have really liked this cup; she used it for drinking water. Now, everyone, take a look at this cup.” Wang Shen’s laser pointer moved to the picture on the right. “I took this picture of the cup in the deceased’s office. It was placed next to a flowerpot, and you can see there’s still some soil on the rim. So I guess the deceased used this cup to water his plants. A pair of matching cups, one treasured by the woman, the other discarded by the man. There must have been some kind of emotional entanglement between them. This entanglement could very well have been the secretary’s motive for the murder. Moreover, the information I have is that the deceased’s secretary was his junior in college and had been pursuing him since then.”