Adolescents' cognition is influenced by a dynamic educational environment. Studies examining the influence of schools, classrooms, and teachers often overlook the momentary variation found in these environments and the effect this variation has on student cognition. Using an achievement goal theory framework, this study examined the momentary variation of goal structures in high school science classrooms and their influence on students' cognitive engagement using objective observations and the Experience Sampling Method, a unique data collection method that elicits students' momentary subjective experiences related to the momentary classroom goal context. The results of this study indicate that goal structures significantly vary from moment to moment and have a considerable impact on student cognition substantiating the concept of momentary goal structures.