Changes in DNA base-repair may serve as an indicator of the time elapsed since divergence from a common ancestor. DNA sequences can now be analyzed. The simulation presented in this article allows students to observe the accumulation of changes in a randomly mutating sequence of playing cards. The cards are analogous to DNA nucleotide or protein amino acid sequences. In this simulation, the accumulation of mutations is solely the result of random drift; there is no natural selection. Although the simulation does not address all aspects of, nor complications in interpretation of, a molecular clock, students learn the general concept. (Contains 10 figures, 1 table, and 7 resources.)