I have a question about the MS renormalization scheme. When you choose this scheme, all sorts of strange things start happening. The mass in your Lagrangian can no longer be the physical mass. The 4-momentum of a physical particle squares to the physical mass, not the free-field mass. But what I don't get is why the creation and annihilation operators get divided by the square root of the residue of the propagator at the physical mass. How does this happen?