The small-world theory says that everyone in the world is connected through a short chain (链条) of people they know. Another name of this chain is degrees of separation. Each degree is a step that separates a person from someone he or she does not know. There are zero degrees of separation between a person and the people he or she knows directly. This means that there are zero degrees between Ben and his mom. There is one degree of separation when just one person separates someone from a person he or she does not know. Ben knows his mom, but he hasn’t met her close friend Amy. Therefore, Ben is one degree away from Amy. As the chain continues, Amy’s uncle, Mark, is two degrees. John Carter is three degrees, and his father is four degrees. Dr. King is five degrees away from Ben. The theory says that there are no more than six degrees of separation between any two people in the world.

　　The small-world theory has a lot to do with math. The theory assumes (假定) that each person knows 100 people. Each one of those 100 people knows 50 different people. Each of those 50 people knows another 50 people, and so on. When someone continues the calculation to six degrees, it is like this: 100 × 50 × 50 × 50 × 50 × 50 = 31.25 × 109. This number is greater than 31 billion (a billion = a thousand million). There are over 6 billion people in the world. These numbers show it is possible that six degrees of separation could include everyone in the world.

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