Mental set represents a form of rigidity in which an individual behaves or believes in a certain manner. In the field of psychology, this effect has typically been examined in the process of problem solving and specifically refers to the brain’s tendency to stick with the most familiar solution and to stubbornly ignore alternatives, or people cannot quickly switch rules and tasks according to the situation or demands. One of the most famous examples is the so-called water jar problem, which was originally developed by Luchins. Eye tracking technology revealed that the cognitive mechanism underlying this phenomenon was attentional bias. Previous knowledge likely directs attention toward relevant information and away from irrelevant information. Thus, the familiar solution that is consistent with the already activated knowledge is more likely to be chosen. According to which kinds of knowledge affected the problem-solving activities, we can distinguish the mental set derived from declarative knowledge and the mental set derived from procedural knowledge. For the mental set that is driven by prior knowledge, particularly expertise in a domain, both prior knowledge and a similar problem were generally considered the necessary factors to the perseveration of the mental set. Mental set always occurs when people are confronted with a problem situation that is similar to previously experienced problems. If a problem situation is different from previous experiences, then no cues will elicit retrieval of previously acquired knowledge and no attentional bias will occur. However, the mental set is also likely strengthened by repeated practice in a short time and can be interpreted as a temporary by-product of procedural learning. In particular, when a solution is used to solve the first problem successfully, it gains activation and, as such, is more likely to be used to solve the second problem in the set, in turn gaining additional activation, and so on. After repeated practice, the solution that is satisfactory for all of the practice problems gradually realizes mechanization and is likely to be automatically activated in similar consecutive problems. Consequently, problem solvers tend to solve similar problems in a fixed way and become faster and faster in behavior performance. Meanwhile, mechanization of a particular solution decreased cognitive flexibility, which likely increased the switching costs from the practiced problems to a totally different problem. Therefore, perseveration of the mental set was independent of the similarity between the problems. Regardless of whether the next problem is similar to the previously practiced problems, problem solving will be hindered when people try to explore alternative solutions rather than using the repeatedly reinforced solution.