Background: As prospective teachers traverse the early stages of their careers, their first experiences of teaching (both in-course and in-classroom), and the transitions between them, need to support the growth of their professional learning. In the context of STEM education, more needs to be understood about pre-service teachers' lived experiences of these initial sessions, and the transitions between them, so that teacher educators can better prepare and support pre-service teachers to learn from these experiences and advance along their professional pathways. Purpose: This study sought to explore primary teachers' lived experiences of teaching science for the first time, firstly within a science teaching methods course, and then within a practicum. Methods: Participants were 24 pre-service primary teachers attending a primary school teacher preparation programme in the USA. A phenomenological approach was adopted to describe the essence of the pre-service teachers' lived experiences in teaching science for the first time, initially in a science teaching methods course and then in a practicum. They were interviewed at two points in time: once after their micro-teaching on the methods course, and then again after they had taught science at least once in their practicum. Data were analysed qualitatively. Findings: The analysis suggested that the essence of pre-service primary teachers' first-time science teaching experiences appeared to be about "constructing" 'operating systems' for teaching science (in the methods course), and then, once they had transitioned to the practicum, "reconfiguring" their operating systems, in part to align with the mentor teachers' conceptualisation of what constituted science instruction and learning. Conclusion: These findings highlight how the notion of 'experience as teacher' could assist science teacher educators in helping preservice primary teachers to adjust their expectations and support their learning as they transition through the early stages of their STEM teaching journeys.