Latest studies on situational interest (SI) point out that the expressed situational interest (ESI) and observed situational interest (OSI) of students that emerge during science courses differentiate from each other. However, the studies aiming to determine this difference are rare. Accordingly, the purpose of this qualitative case study is to compare the ESI and OSI of middle school students emerging during a science course and determine what type of sources these students show in relation to their SI changes. Turkish six-graders (N= 30, age 11-12 years) participated in a heat transfer course, and this course was video-recorded. The SI of the students was measured repeatedly in different sequences and periods of the course and examined by combining student self-evaluations and observations from the video recordings. As a result of the study, it was observed that although the SI of the students increased especially during hands-on activities, it decreased in general towards the end of the course, ESI and OSI differentiated with time, and the prominent sources included novelty, personal relevance, concreteness and engagement.