“The pattern of many connections within one tight community, punctuated by occasional ties to distant communities, describes a vast range of systems. Neurons in the brain mostly connect within one cluster, but occasionally their axons extend far outside, to an entirely different cluster. Proteins in a cell mostly interact within one functional group, but occasionally they connect with receptors far removed. Sites on the internet mostly connect within one tight group (celebrity news sites link to other celebrity news sites; biology sites link to other biology sites), but occasionally a site will connect far outside its cluster (TMZ will link to a study on neuroscience). The Kevin Bacon game had shown that there are surprisingly few steps between any two nodes (actors) in these kinds of networks. So Watts and Strogatz called a system with mostly local connections but occasional distant ties a “small-world network.”