Common Sense

What is common sense? The term is thrown around loosely. Those who think they have it often express condemnation for those they think do not. How do you "get" it? Nature vs. Nurture? or does learning mathematical and scientific equations create it? Ummmm. I like to think I have common sense but to admit that I do almost makes me seem base. I consider myself intelligent (according to tests as reliable as they may/may not be-top 2% of the human population-not sure where I rate in the animal world) and I have great intuition and observation abilities, but when I make the comment "It's common sense" I realize that others do not recognize common sense, so I am now on a quest to define it. I do believe that common sense is indeed most UNcommon.