The problem in this study was to determine the process of acquisition of some fundamental notions (structures) of space and time in preschool children. A fundamental structure that underlies the idea of space is the opposition between the permeability and the impermeability of a physical body for another physical body. A fundamental structure on which the concept of time is based is the opposition between reversibility and irreversibility of complex processes. In two experiments, children aged 4, 5 and 6 years were placed in situations in which they were free to reveal their beliefs in the permeability of a solid body (a glass wall of a box) and in the reversibility of complex processes (turning back into a little boy or girl again). The results showed that at the beginning of the experiment almost all the children denied that permeability or reversibility could occur in real life. However, under the influence of a fairy tale and an adult's instruction, the majority of 4‐ and 5‐year ‐olds revealed their credulity towards such unusual properties of space and time both at the level of practical actions and at the level of verbal judge