In this study, effects of 2 physical manipulative instructions on students' geometrical knowledge acquisition were compared. Geometry topics' presentation was conducted via using physical manipulative and the practice stage in two different forms: (1) practice consisting of problems supported with explicit metacognitive questions and (2) practice consisting of problems not supported with explicit metacognitive questions. Participants were 220 6th grade students, studying in 5 classrooms. Students' knowledge acquisitions were tested by administering knowledge tests before and after the instruction. Furthermore, follow-up interviews were conducted with randomly selected students according to their achievement levels to get their views about the effect of manipulative instruction on their geometrical knowledge acquisition. The results indicated that the two instructions were equally effective in promoting geometrical knowledge acquisition. Metacognitive questions together with manipulative use and group work seemed to help partly students' knowledge acquisition. Hence, active involvement, both physically and cognitively, seemed to play a crucial role.