In a perfectly periodic solid and at absolute zero, where the electrons do not suffer any scattering, the electrical conductivity is infinite. However, pure periodic solids also display the curious phenomenon of Bloch oscillation where the current oscillates with the time when a dc voltage is applied. This has been observed in experiments. But how can the infinite conductivity is compatible with the phenomenon of Bloch oscillation? Bloch oscillation must mean that the current response is finite for a finite voltage.