A nuclear explosion results when a fission chain reaction mediated by neutrons is initiated in a sufficiently large lump, called a super-critical mass, of nearly pure 235U or 239Pu. As a super-critical mass is inherently unstable it has to be created just prior to the explosion. This can be done either by the gun method, in which two sub-critical mass pieces are brought together by firing one piece into the other, or by the implosion method, in which the density of a single sub-critical mass lump is raised. The increase in density is achieved by detonating conventional explosives to compress a sphere of the fissionable material. The implosion method works because the critical mass decreases with increasing density, so sufficient compression will convert a sub-critical into a super-critical mass.