The hideous neutron transport equation has been reduced to a simple one-liner neutron diffusion equation. Everyone breathes a sigh of relief as it is shown to be very solvable, and a criticality relation (a balance between neutrons created and destroyed) links the geometry of a reactor to its material of construction. Different geometrical examples (slab, cube, cylinder, sphere) of reactors are introduced as real examples of designing a nuclear reactor to support a fission chain reaction.