This ethnographic research examines how children enact developing understandings in science through multiple interactions. Grounded in sociocultural theoretical frameworks, we consider learning to be a social, cultural practice, with understandings as co-constructed between participants through talk and in interactions. With these underpinning frameworks, we have explored water activities in kindergarten and examined how children and teachers collaboratively constructed science investigations to explore questions as they emerged from open-ended activities. The analysis revealed how children's investigations were mediated by their own speculations and explanations. Our primary claim herein is that children's questions, speculations, and insights were used collaboratively by teachers and children, and as such, became a structure in this classroom that supported children in taking agency. In this process, science curricula and working theories on science phenomena were generated. This was facilitated by teachers' openness to emergent approaches for their science curriculum. Through a discussion of these claims, an emphasis is placed on the value of students being positioned as co-constructors of science curricula. Furthermore, the integral role of the teacher in emergent curricula is introduced and developed as critical for being responsive to students' interests and insights.