The simulation demonstrates the reactions of nuclear fusion. A fusion is the spatial type of reactions for light nucleus. Height temperature (order to 107 - 109 ?K) is requiring for initiating such reaction because it needs for nuclei to have kinetic energy enough to overcome coulomb potential barrier. But the fusion may be self-sustaining thanks to considerable energy releasing when two light nuclei are combined to form a heavier nucleus. So they are called thermonuclear fusion reactions. Probably thermonuclear reactions are the primary energy source for stars. In this simulation you may study five reactions. After choosing the type of reaction (with switches from the right) you may see reaction formula under picture window. The sliders below simulation serve to incoming particle´s velocity assignment and as a result their kinetic energy assignment. Because the reactions can take place only at very high speeds so as the velocity unit is accepting velocity of light in vacuum c. If total energy of both particles is sufficient for beginning fusion you will look a result of reaction on picture. Otherwise you will observe their scattering. Analyze the values of rest masse and kinetic energy for all particles, which are traced in next windows. You may note that the total mass in the system has changed. Remember Einstein´s relation between mass and energy E = mc2, and you understand the reason energy releasing in nuclear fusion. It is a transformation mass-energy into kinetic energy! By summing the kinetic energies before and after the reaction you should be able to verify this result.