Figure 1: Kiptily and colleagues [3] observed fusion reactions in a donut-shaped vessel called a tokamak (gray). The red, blue, and pink balls represent protons, neutrons, and electrons, respectively. In each fusion reaction, a deuterium nucleus (one proton plus one neutron) merged with a tritium nucleus (one proton plus two neutrons) to generate a free neutron and an alpha particle (two protons plus two neutrons). The free neutron escaped the tokamak, whereas the alpha particle remained inside the vessel and heated electrons that spiraled along magnetic-field lines (green).