Six_degrees-of-separation

Six degrees of separation is the idea that all people are six or fewer social connections away from each other. As a result, a chain of "friend of a friend" statements can be made to connect any two people in a maximum of six steps. It is also known as the six handshakes rule.[1] Mathematically it means that a person shaking hands with 30 people, and then those 30 shaking hands with 30 other people, would after repeating this 6 times allow every person in a population as large as the United States to have shaken hands (7 times for the whole world). © Wikipedia