The affective domain and its impact on science achievement has been studied with increasing attention over the past decade. However, there has been little empirical work on how individual interest, and facilitation of situational interest in the classroom, facilitates learning of concepts within fine-grained units of instruction. Drawing upon previous theoretical and empirical work, we test the hypothesis that individual interest facilitates situational interest, and that this in turn facilitates content learning, with respect to a 10-day health education intervention focused on teaching middle school students about light, color, and vision. We found that individual interest in science facilitates situational interest in the classroom activities and the teacher's instruction, and that situational interest has a positive and significant effect on learning concepts related to light and vision. Our data suggest that facilitating situational interest is a requirement if individual interest and prior knowledge are to be effective scaffolds for conceptual learning. However, targeting of other affective traits like motivation and engagement or cognitive processes like goal setting may be necessary to fully harness students' interest toward effective scaffolding of learning.