A mental model is a kind of internal symbol or representation of external reality, hypothesized to play a major role in cognition and decision-making. Once formed, mental models may replace carefully considered analysis as a means of conserving time and energy.
A simple example is the mental model of a wild animal as dangerous: upon encountering a raccoon or a snake, one who holds this model will likely retreat from the animal as if by reflex. Retreat is the result of the application of the mental model, and would probably not be the immediate reaction of one whose mental model of wild animals was formed solely from experience with similar stuffed toy animals, or who had not yet formed any mental models about wild raccoons or snakes.According to the theory of mental model we are saving all perceived stimulations and observations to the memory in the form of model , which is designed on the basis sensual information and is combined with already saved informations. Prof. Jay Wright Forrester defined mental model as: "The image of the world around us, which carry in our head, is just a model. Nobody in his head imagin all over the world, government or country. He has only selected concepts and relationships between them and those used to represent the real system.