Common sense is the basic human ability to think. Common sense has nothing to do with knowledge that a human being possesses. Common sense is actually a measure of one's IQ level Common sense is an in-born trait in some cases, while it can be developed and harnessed with time too. People with a good common sense are sharper and quicker at solving puzzles, riddles, mind-boggling games and other related stuff which requires great mental sharpness and have nothing to do with your knowledge levels.
Common sense is not something that everyone can claim to have; it is a rare quality of mind. It is something that people is really endowed with, being a special capability of your brain. In the words of Voltaire," Common Sense is not so common." The word "common" here is a misnomer since this particular capability is not that commonly seen in human beings.