I grew up playing war. We threw dirt and rocks at each other. We'd lead attacks. We'd break up into squads. It became a neighborhood thing for a while, our neighborhood against the other neighborhood. There was always a war breaking out somewhere.
Where I lived, it was a cold mining place, a village called Dunston. The only time you saw a Rolls-Royce was when somebody died.
The fundamental point in fabricating a chain reacting machine is of course to see to it that each fission produces a certain number of neutrons and some of these neutrons will again produce fission.