This is the "six degrees of separation" theory. This theory originated from the experiment conducted by sociologist stanleyMlgram in the 1960s: in the experiment, he asked each participant to try to send a letter to a "target person" living near Boston, and stipulated that each participant could only forward it to one person they knew, so as to see how many hands were needed to achieve the goal. Finally, according to the experimental results, he came to a conclusion that any two strangers can establish contact through "relatives and friends", and the number of relatives and friends between these two people is about 5. In other words, the distance between a person and another stranger will not exceed six, and you can contact any stranger through six people at most.