This study has been performed in Sweden, where the preschool curriculum states that children's understanding of simple chemical processes is a goal to strive towards [13]. However, uncertainty within the current preschool practice exists and has been described by B. Sundberg et al. [20]. Motivated by the lack of scientific literature on what chemistry content is suitable for preschool children and how to introduce it, this study aims to tackle how abstract concepts like "atoms" and "molecules" can be introduced to preschool children. With this purpose, a play-based learning intervention was designed, following the cultural- historical model for preschool science education proposed by M. Fleer [7], and implemented in two Swedish preschools, dividing a total of 20 three-years-old children into four groups of five children each. Data were collected in the form of video-recordings of the sessions and analysed following the principles from the experimental-genetic method summarized by N. Veresov [23]. Results are presented in the form of vignettes that illustrate significant moments from the intervention, together with discussion of how the social situation of development, the zone of actual development and the mediating tools facilitate the children in starting to talk about atoms.