We all know what "rationality" is, or think we do, but for concreteness here is the (current as of this writing) first paragraph from the Wikipedia article:
In philosophy, rationality is the characteristic of any action, belief, or desire, that makes their choice optimal under a set of constraints. It is a normative concept of reasoning in the sense that rational people should derive conclusions in a consistent way given the information at disposal. It refers to the conformity of one's beliefs with one's reasons to believe, or with one's actions with one's reasons for action. However, the term "rationality" tends to be used differently in different disciplines, including specialized discussions of economics, sociology, psychology, evolutionary biology and political science. A rational decision is one that is not just reasoned, but is also optimal for achieving a goal or solving a problem.
A little further on it mentions that the "idealized form of rationality is best exemplified by computers".
But what if your goal or problem is already solved? What is the rational course of action in the absence of unsolved problems?