What do the metaphors do/achieve in everyday language activities? What function do the conventional metaphors have? This paper described the most fundamental function which the metaphors ("X is Y") do/achieves in everyday language activities. In many studies of the metaphor, it is assumed that the metaphors have so-called "metaphorical meanings" different from literal meaning of the expression. However, there is no evidence that support such premise. This article argued that the metaphors only mean literally. The "metaphorical meaning" is not the meaning of the expression, but evoked by the receiver who accepted the expression "X is Y" literally. Thus the most fundamental function of the metaphors is forcing the receiver to assume <X is Y> with the expression "X is Y". The difference of classification of X and Y produces the feeling which distinguishes metaphors from other expression. The conventional metaphors, which often unnoticed as metaphor, force tacitly so that the members of the society see and treat X as Y. In the society, most part of reality is told by language activities. In such activities, the metaphors offer useful ways of objectivization required for the interactive activity in the society. When he/she became to be able to use the metaphors, the receiver is accepted as a member of the society who shares the reality.