EG and CG students’ conceptual changes were identified via four two-tier questions which were taken from “Determining Differentiation in Conceptual Structure Test (DDCST)” prepared by Şahin (2010) including 16 questions. The test’s reliability coefficient (Cronbach’s Alpha) was computed as 0,81. Because the data obtained from the test did not show normal distribution and the questions used in the test were included in the classified scale, the test’s validity was determined by a hypothesis test. Expert opinions were taken for the construct validity. The first-tier of each item consists of a content question having four choices; the second tier explores students’ reasons for their choices made in the first tier. The two-tier questions are presented in Appendix A. The first question was asked in order to probe the understanding of the students about the relationship between the effect of fluid buoyancy force and gas buoyancy force on standing and moving objects or floating and sinking objects. The second question was searching for the relationship among fluid buoyancy force, submerged volume of the object and the amount of displaced fluid concepts. The third question was asked to investigate the students’ understanding of the relationship among fluid buoyancy force, the density of fluid, and sinking and floating concepts. The fourth question was asked to probe the students’ understandings about the relationship among gas buoyancy force, volume of object and gas pressure concepts.