Just as an ordinary fire needs oxygen, so does an atomic fire require the tremendously powerful atomic particles known as neutrons. Unlike oxygen, however, neutrons do not exist in a free state in nature. Their habitat is the nuclei, or hearts, of the atoms. How, then, does the spontaneous combustion of the critical mass of U-235 or plutonium begin? All we need is a single neutron to start things going, and this one neutron may be supplied in one of several ways. It can come from the nucleus of an atom in the atmosphere, or inside the bomb, shattered by a powerful cosmic ray that comes from outside the earth. Or the emanation from some radioactive element in the atmosphere, or from one introduced into the body of the bomb, may split the first U-235 or plutonium atom, knock out two neutrons, and thus start a chain reaction of self-multiplying neutrons. To understand the chain reaction requires only a little arithmetic. The first atom split releases, on the average, two neutrons, which split two atoms, which release four neutrons, which split four atoms, which release eight neutrons, and so on, in a geometric progression that, as can be seen, doubles itself at each successive step. Arithmetic shows that anything that is multiplied by two at every step will reach a 1,000 (in round numbers) in the first ten steps, and will multiply itself by a 1,000 at every ten steps thereafter, reaching a million in twenty steps, a billion in thirty, a trillion in forty, and so on. It can thus be seen that after seventy generations of self-multiplying neutrons the astronomical figure of two billion trillion (2 followed by 21 zeros) atoms have been split. At this point let us hold our breath and get set to believe what at first glance may appear to be unbelievable. The time it takes to split these two billion trillion atoms is no more than one millionth of a second (one microsecond). If we keep this time element in mind we can arrive at a clear understanding of the tremendous problem involved in exploding an A- or an H-bomb. And while we are recovering from the first shock we may as well get set for another. That unimaginable figure of two billion trillion atoms represents the splitting (explosion) of no more than one gram (1/28th of an ounce) of U-235, or plutonium.