Studies have shown that students’ awareness of the goals and purposes of the laboratory activityis important for their possibility to participate in and learn from the activity. While practical activitiesoften have been considered to be a central part of science education, relatively few studies haveexamined laboratory work in situ. In this paper we addressed these issues by examining (a) whatpurposes are distinguished when students’ work with a laboratory assignment and (b) how thesepurposes are made continuous with the teacher’s aim with the assignment. The data was based onclassroom observations from two ordinary laboratory settings, one from a chemistry class in lowersecondary school and one from a physics class in the natural science programme in upper secondaryschool. Although both student groups acknowledged their teacher’s intentions with the practical andcould act towards the more student centered purposes of the activity, e.g. describe what happens withthe copper and measure the speed of a small vessel respectively, there were differences regarding thepossibilities the students had to act toward the activity’s final aim. The results showed that these factorscan be referred to the amount of purposes introduced by the teacher as well as those that arosebecause of contingences, and the connection of these purposes to students’ prior experiences.