A genuine interest in science is an important part of scientific literacy, and thus a critical goal for science education. Recent studies, however, have found that school science has not been effective in meeting this goal, an important reason for which is the lack of knowledge about what makes science interesting (or not) to the students. Using instructional episodes as the unit of analysis, this study investigated the effects of learning environment elements (content topic, activity, and learning goal) on student interest in science. The findings indicated that when judging the interestingness of an instructional episode, students focused primarily on the form of activity rather than content topic and learning goal. Activities that were "hands-on" in nature and allowed for engagement with technology elicited higher interest. This study highlights the need to place more emphasis on the role of activity in constructing interesting learning environments, and in the mean time, suggests that student science interest could be improved by making changes to relatively easy-to-manipulate aspects of learning environments, such as those examined in the study. (Contains 3 figures and 7 tables.)