To indicate how nuclei attract each other if they come close enough together.
Two small horseshoe magnets.
Let the magnets snap together as you discuss how they do work and in the process become slightly less massive, as in E=mc^2. Then discuss the analogy to protons combining to form deuterium and then helium while releasing energy.
E. Kashy and D. A. Johnson, A model to illustrate forces in nuclear fusion, AJP 62, 804-806 (1994). Lawrence Ruby, Letter: Fusion Simulator, AJP 63, 107 (1995).