Part of "the knowledge necessary to make a nuclear weapon" 1. Atomic nuclei of uranium or plutonium can decay and set free several NEUTRONS and ENERGY. 2. If one of these neutrons hits (better: "is captured") by another nucleus of uranium or plutonium, a fusion takes place: the resulting heavier nucleus is instable, disintegrates and sets free NEW NEUTRONS and ENERGY. And So On. This is how a nuclear chain reaction works. BUT: 3. Neutrons (just like atomic nuclei) are small compared to the hole atoms (nuclei plus electron shell) in such a mass: they're like "grains of rice on a soccer field" and thus many neutrons leave the mass and don't trigger anything. So, only if the clump of uranium or plutonium is big enough (a "critical mass" or more) sufficiently many neutrons are captured and the chain reaction can take place. On triggering a bomb, smaller ("subcritical") masses are combined to form a supercritical mass: the chain reaction begins and destructive amounts of energy are freed
Libya rival government calls for regional conference to tackle migrant influx (Reuters)
from the article: Bush warns nuclear Iran could spur 'World War III'