Fog surrounds one when one least expects it. It disappears when one thinks one is about to clutch it. It is a locus of numerous metaphors and it is endowed with the greatest of all powers : the power to transform whatever it hides and whatever it reveals. If, in scientific terms, we have a firm grasp of the composition of fog, what can we say about its representation in our collective imagination ? Understood in terms of its ambivalence, fog is a power which, in the course of our survey carried out with schoolchildren and with adults, is characterised as something that is both benevolent and malevolent. It is both a protective cocoon and the locus of alldangers. As something that is amorphous and ever-changing, fog allows us to dream and to project upon its screen all the creations of our imaginations.