process, but only after the
             nucleus temporarily increases its' mass number by one. Two atoms, P-239 and U-235,
             undergo this type of division and release energy at the same time. U-235 emits two or
             three neutrons in the process, while P-239 emits many more. Either of these two atoms
             may be used in an atomic bomb. After absorbing a neutron, an atom of these elements
             emits several more neutrons, making a chain reaction possible. If the surrounding
             structure is properly designed, the result is an explosion.
             The amount of fissionable material needed to make an explosion is called the
             critical mass, or the trigger quantity. Because a chain reaction would begin immediately,
             the material cannot be place all together. It must be broken up and contained in pieces
             that are smaller than the critical size. These pieces are then brought together in one
             supercritical lump, and at the moment of detonation, neutrons are fired into it. Although
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