As the Los Alamos scientists approached the design of the uranium-based bomb, one of their main problems was how to prevent it from exploding prematurely. It takes only a small amount of uranium, about the size of a baseball, to produce a critical mass resulting in a tremendous nuclear explosion. The problem was how to bring together the critical mass at the right moment. The scientists solved this problem by using a gun barrel inside the bomb to fire a bullet of sub critical uranium at a target of subcritical uranium. The rapid impact of the two masses would make a critical mass large enough to produce a chain reaction and a nuclear explosion. The resulting bomb that used this bullet method was called "Little Boy" because its internal gun barrel made it long and thin. The gun barrel...