I don't understand...

 It's another late night, so another quickie (that I'll probably expand into a longer one later). What does it mean to know or understand something? Last night, I said to my friend that I didn't know how to play Checkers, but I'd play a game with him anyway. I won, because I know the rules of Checkers, but I said I didn't know the strategy or whatever. I think there's an interesting break between what people mean when they say they know or understand something. Some people would say you only know or understand something if you know all there is to know about it. I remember reading an article once that basically said that the best person in the world sets the bar for "good", the top 50 or so are "okay", and anyone below that is a clueless idiot. Other people say if you know how the checkers move, you know how to play Checkers.