According to Wikipedia, "Common sense is a basic ability to perceive, understand, and judge things, which is shared by ("common to") nearly all people and can reasonably be expected of nearly all people without any need for debate." As is the case with many other qualities, it may vary from person to person. Even the development of the artificial intelligence has gone so far, that we are starting to ask the machines to have "common sense". But let's not talk about the artificial intelligence.
We are born with common sense, which is meant to keep ourselves safe from harm. It helps us or helps us assert correctly situations and people, and makes our brain go overdrive when facing a danger.
Take just now this fun quiz to see how much common sense you have