From this story, we can see that Cao Cao is a man who understands psychology. His brilliant idea, in modern psychological terms, is the application of conditioned reflex.
Plum is a special fruit in China, which has been planted for more than 3000 years. Plum is rich in organic acids, such as tartaric acid, tannic acid and malic acid, so it tastes sour. Eating plums can quench thirst, clear away heat and reduce fever. Everyone who has eaten plums has had such an experience. The sour taste of plums stimulates oral saliva secretion and leaves a deep impression on people's cerebral cortex. Therefore, when people are thirsty in the future, as long as they see plums, even hear plums, and think of plums, they will associate their sour taste and cause saliva secretion, which is a manifestation of conditioned reflex.
Of course, if a person hasn't eaten plums and doesn't know what plums are, even if a big basket of plums is put in front of him, it won't make him drool. Because he knows nothing about plums, he will not form conditions. After seeing plums, the cerebral cortex will not be excited, and the natural salivary glands will not be excited, causing increased secretion.
However, although there is scientific truth in looking at plums to quench thirst, "looking at plums" is only a kind of self-comfort, which can only be used for temporary needs, and ultimately cannot really "quench thirst". Therefore, people later used the idiom "looking at plums to quench their thirst" to describe that their wishes could not be realized, so they had to comfort themselves with fantasy.
There is also an idiom similar to "looking at plums to quench thirst", which is also a metaphor for comforting yourself with fantasy. Cao Cao used Merlin as an excuse to quench the thirst of soldiers, and the saliva produced in soldiers' mouths used conditioned reflex. So can "painting cakes" really satisfy hunger? It stands to reason that when people see painting cakes, the cerebral cortex is stimulated due to conditioned reflex, which will also lead to the secretion of saliva, but saliva can not satisfy hunger. The result is self-defeating, but it stimulates gastrointestinal peristalsis and deepens hunger. However, Dr. Rodin, a professor of psychology at Yale University in the United States, made an interesting experiment and proved that "painting cakes" can also satisfy people's hunger.
Dr Rodin invited four volunteers to participate in the experiment. First, they didn't eat for 0/8 hours, and the average blood insulin content was 292 pg/ml. Then, the doctor asked someone to bring the painted food and a large pot of delicious beef patties that had just been fried. Those subjects salivate at the sight, but they can only see and smell, but they can't eat. Immediately, the doctor took a few drops of blood from everyone, and the test results showed that the insulin content in the blood of these four subjects increased sharply, reaching an average of 892 pg/ml. Because glucose in the body needs more insulin stimulation to enter the adipocyte membrane, the increase of insulin can convert more sugar into fat and make more sugar inhaled into the body. So "seeing cakes" and "smelling cakes" can really make people hungry and even gain weight! It is often said that ten cooks make nine fatters, but chefs generally don't eat much. From here, we may find the answer: their obesity is not necessarily what they eat, but what they smell.
Now, some scientists are engaged in this research, hoping that when people are thirsty or hungry, they can meet their needs by "seeing" or "smelling" food, so that people can eat less. If there is such a day, "looking at plums to quench thirst" and "painting cakes to satisfy hunger" will really be "worthy of the name".