Molecular similarity is one of the strongest lines of evidence for evolution--and one of the most difficult for students to grasp. That is because the underlying observations--that identical mutations are found in closely related species and the degree of similarity decreases with evolutionary distance--are not visible to the human eye. And it's often expressed in tables showing numbers of mutations that do not show how these patterns came about. The activity described in this article helps make this process visible for students. It simulates mutations occurring in a diversifying lineage and asks them to reconstruct the lineage. Because students participate in the process of divergence, they gain a better understanding of how it works and what scientists are doing when they construct a molecular phylogeny, or phylogenetic tree--a structure that shows how a group of species branches from a common ancestor. This activity can be used with advanced students to demonstrate other concepts, such as the molecular clock--an estimate of how long different species have been separated based on mutations. And all this can be done without any special materials. (Contains 4 figures.)