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We investigate the generation mechanism of high harmonics from the bulk of a ZnO crystal driven by intense mid-IR laser pulses by perturbing it with a weak (< l0-5) second harmonic field.We find that high harmonics are emitted upon recollision of the electron with the hole.Because the same mechanism underlies high harmonic generation in atoms and molecules, our findings allow to extend well developed attosecond techniques to solids.Future experiments should permit us to tomographically image the band structure of a solid and the orbital structure of excitonic states.