Fission was first realized by the Germans in 1938. During this time Adolf Hitler was preparing for global domination. Many feared that he would harness the energy created during fission and use it as a weapon. To prevent this, the United States began work on their own fission bomb under the code name Manhattan Project. As history dictates, the Americans became the first country to produce the fission or atomic bomb.
Fission is the splitting of a nucleus within an atom. When fission occurs the nucleus splits into several smaller fragments. The fragments that are produced (usually two) are equal to about half the mass of the original nucleus. In addition to the new fragments, two or three neutrons are also released. However, the sum of the neutrons and the new fragments is not equal to the mass of the original nucleus. They actually weigh slightly less than the original nucleus. The reason for this is that some of the original mass is converted into energy.
In rare cases, fission can occur spontaneously. The scientist involved in the Manhattan Project were able to make fission occur by forcing a heavy atom to capture an extra neutron in its nucleus. Most natural elements are very stable and extremely difficult to split. So far only one element has been found to be fairly easy to split - Uranium. Uranium atoms are very large compared to most atoms and it contains more neutrons than protons.
So how exactly does fission occur then? After scientist found that Uranium had large atoms, it became simple. All they had to do was shoot a neutron at the element. The neutron would eventually collide with a nucleus, force its way in, and cause the atom to split. Once the atom splits, it shoots out more neutrons which will split more atoms thus causing a chain reaction. After the neutron has been fired at the Uranium, the whole chain reaction usually takes less than one second to finish. To emphasize how much energy this is remember that the first atomic bomb only used 1/10th of 1 percent of its potential energy yet it still killed more than 60,000 people.