This article studies an activity in a Swedish preschool setting where children's elaborations and understandings of floating and sinking are central. In a Swedish preschool context, it is advocated by the National Agency for Education (2016) that different forms of knowledge and ways of learning are used within the institutions to form a coherent whole. Based on a cultural-historical framework (Fleer, 2010; Hedegaard, 2012; Vygotsky, 1978, 1987), it is argued that if children are supported by a teacher, who mediates a cultural and historical, as well as institutional, perspective of knowledge, children may enhance their knowledge of science. Data was collected using a video-based technique called shadowing (Czarniawska 2007), and the overall study was framed within a case-study methodology. The analyzed activity show four children collaborating and exploring a range of aspects related to floating and sinking together with one teacher, acted out in a playful but focused manner. The results show that aspects such as the item, the fluid, and the way children handled the items to remove or add weight, were found important by the children. They used everyday language to talk about size, holes, weight, amount of water, and what changing preconditions would mean when different objects were placed in water. Their vocabularies were enhanced during the activity and seemed to foster emergent notions of density and Archimedes' principle, indicating that the language used has the potential to mediate the progress of both spontaneous and scientific concepts, where scientific concepts are understood as emergent.