Rationality is the recognition of the fact that nothing can alter the truth and nothing can take precedence over that act of perceiving it.
Not to be absolutely certain is, I think, one of the essential things in rationality.
The irrationality of a thing is no argument against its existence, rather a condition of it.
One might equate growing up with a mistrust of words. A mature person trusts his eyes more than his ears. Irrationality often manifests itself in upholding the word against the evidence of the eyes. Children, savages and true believers remember far less what they have seen than what they have heard.