|How do they split the first atom in an atomic bomb? — N, Houston, Texas
Once the bomb has assembled a super-critical mass of fissionable material, each chain reaction that occurs will grow exponentially with time and lead to a catastrophic release of energy. But you're right in wondering just what starts those chain reactions. The answer is natural radioactivity from a trigger material. While the nuclear fuel's own radioactivity could provide those first few neutrons, it's generally not reliable enough. To make sure that the chain reactions get started properly, most nuclear weapons introduce a highly radioactive neutron-emitting trigger material into the nuclear fuel assembly.