Homogeneous vapor-to-liquid nucleation takes place when the water molecules in the gaseous phase undergoes condensation, resulting in the formation of nuclei of water droplets in the liquid phase.
This happens spontaneously and without interactions with other materials. Homogeneous nucleation takes place at random points that are randomly distributed in the volume.
This kind of nucleation does not occur often, because almost always there are possible points where water molecules interact with another material or surface.
The video shows how homogeneous vapor-to-liquid nucleation occurs, based on molecular dynamic simulation.