In this article we problematize the purpose of teaching science in preschool and the competences preschool teachers need in order to conduct science activities in the classroom. The empirical data were collected through an action research project with five preschool and primary school teachers (K-6). In the first section of this paper we use one situation, a floating-sinking experiment, as an illustration of how two different epistemological perspectives generate different foci on which kind of science teaching competences can be fruitful in preschool settings. In the first perspective, the central goal of science teaching is the development of the children's conceptual understanding. With this perspective, we found that the science activities with children were unsuccessful, because their thoughts about concepts did not develop as expected, the situation even enhanced a "misconception" concerning density. Moreover, the teacher was unsuccessful in supporting the children's conceptual learning. The second perspective uses a feminist approach that scrutinizes science, where we investigate if the floating-sinking activity contributes to a feeling of participation in a scientific context for the children and if so how the teacher promotes this inclusion. This second perspective showed that the children's scientific proficiency benefited from the situation; they had a positive experience with density which was reinforced by the teacher. The children discovered that they had power over their own learning by using an experimental approach. On the basis of these findings, we conclude that there are competences other than subject matter knowledge that are also important when preschool teachers engage children in scientific activities. Through process-oriented work with the teacher group, we identified four concrete skills: paying attention to and using children's previous experiences; capturing unexpected things that happen at the moment they occur; asking questions that challenge the children and that stimulate further investigation; creating a situated presence, that is, "remaining" in the situation and listening to the children and their explanations. We discuss possible ways to move preschool teachers away from their feelings of inadequacy and poor self-confidence in teaching science by reinforcing this kind of pedagogical content knowledge.