Students explore the energy that results from the strong force in nuclei. The activity begins with the unexpected result that the mass of a nucleus is less than the sum of the masses of the protons and neutrons in the nucleus. In Part 2 of the activity, students analyze a graph to determine the ratio of neutrons to protons in stable nuclei. Then, they investigate how fission and fusion of nuclei can be accounted for in terms of the binding energy per nucleon.