Laboratory work is considered essential in promoting students' learning of science and of scientific inquiry. What the students perceive as important to learn from a regular laboratory exercise is probably affected by the teacher's objectives. We study the extent to which one teacher's objectives are fulfilled during lab work, and how teacher-student and student-student interactions contribute to developing learning experiences from the laboratory exercise. Do students encounter opportunities to learn in agreement with the teacher's objectives? This explanatory single case study includes use of a palette of methods, such as pre- and post-interviews, observations and video documentation from an experienced secondary school teacher and her 8th grade (aged 13-14) students' laboratory work. Our results point to the importance of teacher involvement to help students understand what to look for, how to do it and why. Especially teacher-student interactions during lab work seemed to influence what students perceived as important to learn. In the laboratory exercise in this case, the teacher helped the students to observe and to use their observations in their explanations. The lab work included learning experiences other than those addressed by the teacher, and the teacher's intentions were partially fulfilled. Not only what the teacher says, but also how the teacher acts is important to help students understand what to learn from a laboratory exercise.