Ensuring the quality of investigations requires the understanding of procedures by which empirical evidence is obtained. This can be interpreted as becoming aware of and using criteria for evidence in one's mental structure. The question is whether this process can be observed in practice. In two schools for pre-university education where 11th grade students were working in small groups on open investigations in biology, all conversations in class -with or without the teacher participating- of eight groups (17 students) were audio-taped and transcribed. All utterances concerning the quality of investigations (3943 in total) were analyzed using five categories: problematization, description, explanation, generalization and application. Half of the students received written feedback twice, the other half paused their own investigations to carry out four specially designed reflection tasks. Talking about the reflection tasks as well as having the teacher present in conversations about investigations have shown to stimulate the spiral of description, explanation and generalization. Students who did the reflection tasks explained and generalized significantly more than students who did not. Still, the majority of the explanations and generalizations came from the teacher. Implications for the role of reflection and the role of the teacher in developing procedural understanding are discussed. © 2011 The Author(s).