Suppose a social network contains a number of people, each of whom has one of two \opinions" (e.g. a preference for Mac versus PC). Each person is connected with a set of \friends", some of the other people in the network. You can choose any person in the network and let them see the opinions of their friends, and if most of the friends have the same opinion, then the chosen person will change their opinion to the one shared by the majority of their friends. If there's an equal split, you can choose their opinion. Assuming the network is connected, can we always nd a sequence of people so as to ensure that they all end up with the same opinion? What kind of knowledge do I need to solve this problem?