The decreasing number of students who are engaged in science learning has been recognised as a problem. The pre-conditions of engagement and actual engagement were examined using a novel research method to obtain detailed information on Finnish students’ engagement in different situations and to gain a better understanding of this phenomenon. The study’s participants consisted of 68 students (31 girls, 37 boys) from 9th grade and 67 students (46 girls, 21 boys) from 1st grade in upper secondary school. The research aimed to answer the following question: How does Finnish students’ engagement occur in exact and life science lessons? Participants received smartphones equipped with a smartphone application that included an experience sampling method questionnaire. The smartphones were programmed to emit a signal during every science lesson and otherwise randomly during the day (from 8 am to 8 pm). The results reveal that situation and grade had significant effects on students’ pre-conditions of engagement and actual engagement. Our results also show that girls had the highest interest in life science lessons and boys in exact science lessons.