A useful analysis of the simplest pore and capillary systems is conducted in order to pinpoint when adsorption and desorption of water vapor is a reversible process without an hysteresis loop and when it is irreversible and triggered by critical values of relative humidity (RH). On the basis of the Kelvin formula, some critical values of RH have been found to be a function of the size and arrangement of porosities and capillaries. Besides the hysteresis, the geometry of the pore and capillary system may cause air pockets to be entrapped within the body, so that in many cavities, as in the well known ink bottle , the condensation in and external cavity may prevent condensation in the more internal ones. -- AATA