In my first entry on this blog, let me address a topic that constitutes a key point in almost any study on human behavior, namely rationality. The word rational belongs to everyday language, and its meaning depends strongly on the context in which it is used. Even in its technical sense, rationality does not have a single meaning, having different meanings in evolutionary biology, sociology, economics and politics. And even within a particular science, rationality may have different forms: the German sociologist Max Weber distinguished four types of rationality. Just as any other sociological approach, this interpretation has its detractors, among them pragmatists. In this entry, I will try to discuss what could be called rational in a certain number of situations modeled by game theory, and not always coincide with the classical definition of rationality as utility maximization.