A graph is defined by two sets. One set, the nodes, describes a collection of objects, for example biological species, such as human, whale, mountain lion, crow, pigeon, sea gull, Arabian surgeonfish, zebra fish, and Anemonefish. The second set, the edges, comprises pairs of nodes, such as {human, mountain lion}, {pigeon, sea gull}, {Arabian surgeonfish, Anemonefish}. Each pair represents a connection between objects, which has some meaning. For example, two species are related to one another if they belong to the same taxonomic family, or they both live on the same continent, or they are active in the same part of the day, and so on. Each such feature will create a different graph, because different properties create different sets of pairs (edges).