The question of how a metal becoming superconducting expels a magnetic field is addressed. It is argued that the conventional theory of superconductivity has not answered this question despite its obvious importance. We argue that the growth of the superconducting into the normal region and associated expulsion of magnetic field from the superconducting region can only be understood if it is accompanied by motion of charge from the superconducting into the normal region. From a microscopic point of view it is shown that the perfect diamagnetism of superconductors requires that superconducting electrons reside in orbits of spatial extent $2\lambda_L$, with $\lambda_L$ the London penetration depth. Associated with this physics, the spin-orbit interaction of the electron magnetic moment and the positively charged ionic background gives rise to a ``Spin Meissner'' effect, the generation of a macroscopic spin current near the surface of superconductors. We point out that both the Meissner and the Spin Meissner effect can be understood dynamically under the assumption that the superfluid condensate wavefunction $\Psi(\vec{r})$ does not screen itself, just like the $\Psi(\vec{r})$ for an electron in a hydrogen atom. We argue that the conventional theory of superconductivity cannot explain the Meissner effect because it does not contain the physical elements discussed here