Hydrogen recombination at redshifts z ~ 1100 - 1500 leads to the rapid decrease of the optical depth of the Universe due to Thomson scattering. As a result CMB photons previously strongly coupled with baryonic matter become free and able to reach us. The observed acoustic peaks in the CMB angular distribution and the shape of BAOs are formed in the vicinity of the last scattering surface. The question remains how and at which redshifts the observed practically ideal black body spectrum of CMB was formed. Why we dream that proposed space missions like PIXIE will be able to detect traces of any significant energy release in our Universe at redshifts smaller than 2 million? Why spectral distortions can not originate behind black body photosphere or at redshifts higher that 2 million? I plan to describe the nature of the black body photosphere of the Universe, the physical processes leading to formation of the characteristic spectral distortions of CMB and to discuss in detail the unavoidable spectral distortions of CMB due to cosmological recombination, Silk damping and presence of the hot gas in the early Universe. It will be mentioned why we will never observe in the spectrum of CMB the traces of electron-positron annihilation in the early Universe or due to decay of tritium and conversion of Be-7 to Li-7 at z ~ 30 000.