So I tend to here something like "(insert famous scientist) mastered calculus at the age of 13". What exactly is that suppose to mean? Surly you can be good at a subject and get an A in the course, but still be far from a master. To those of you who are pursuing higher education or especially the professors on here, do you think you've truly mastered any subjects? If so, when did you come to realize that you had mastered it.