In the subproject, after a phase of individual knowledge construction, eighth graders create an explanation video about \u201cthe area calculation of the trapezoid\u201d and thus report their state of knowledge. Essential for learning in this context is the activation of cognitive learning processes (activation of prior knowledge and generation of solution strategies) creating video-based explanations (Lachner et al., 2020). In contrast to previous studies, in the project conception subject-specific didactic criteria for explanatory quality are considered. In the problem-solving process of trapezoidal area calculation, the students are encouraged to generate own, different solution approaches (Hu\u00dfmann et al., 2015) and to record their explanations video-aided (\u201cvideo explanation\u201d). In the subsequent teacher-led phase of knowledge construction the students reflect on these approaches and integrate their findings into the target knowledge (various formulae and their references and justifications). Based on previous insights (Lachner et al., 2020), we investigate in which phase (individual phase versus teacher-led phase of knowledge construction) the recording of a video explanation is more conducive to learning. In addition, the effect of different variants of support (creating video explanations) (cf. H\u00fcbner et al., 2010; N\u00fcckles et al., 2020) on the quality of the explanations and on learning success is investigated.<\/p>\n<\/div><\/div><\/div><\/div><\/div><\/div>