In a German out-of-school laboratory, 293 medium-achieving 10th-grade students participated in a lesson unit about gene technology. They were divided into two groups (I-1, I-2), both of which followed the same hands-on lesson procedure. Students within I-2 were additionally confronted with alternative conceptions to central issues of the specific topic. The authors monitored cognitive achievement, mental effort, and calculated the efficiency of instructional conditions. Knowledge scores increased significantly in both groups in the long term. In the short term, I-2 showed significantly better results. Also, these students needed to invest a significantly lower level of mental effort during the interpretation phase. Instructional efficiency differed in the short and the long term. Students within I-2 showed either the same cognitive achievements with lower strain, or higher ones with the same strain. (Contains 7 tables and 3 figures.)