Previous research on science education has described various factors influencing students' participation and produced categorizations of students based on e.g. cultural background. In this article it is argued, theoretically and empirically, that an understanding of students‘ participation in science education needs to begin with an analysis of what activity students are engaged in. The aim is to shed light on student participation in science classroom practice and how altered conditions of classroom practice can make additional space for developing motives for learning science. Activity is conceptualized in a cultural-historical activity theoretical perspective as what transformation of objects students are engaged in. Drawing on an ethnographic study in a Swedish compulsory school, a critical incident of the participation in science education of a 7th grade girl called Helena is analyzed. The results show that altered conditions of classroom practice may produce new possibilities for student participation, and point to the impossibility of determining students as different kinds of students based on a priori categories.