Most cavitating flows are highly complex, their geometry varying rapidly both in time and space. Therefore, one is obliged to study such flows by means of idealised models. In particular, the single bubble model introduced by Rayleigh in 1917 is most useful in describing the general behaviour of the phenomenon in many practical situations. Air may contain vapour while its temperature is lower than the dew point. For triggering condensation and thus causing rain, fine particles of silver iodide are sometimes seeded in the atmosphere. The equilibrium preceding condensation is metastable. Return to stable equilibrium, which does not happen immediately, is brought about by the presence of nuclei of condensation which start the formation of tiny drops. To increase the production of gas it suffices to increase the number of nuclei in the bottle.