The aim of this study is to investigate grade-eight students' conceptions of sinking and floating. Firstly, semi-structured interviews were conducted with 12 students to determine students' difficulties and to develop a multiple-choice diagnostic test. In designing the content of the interview questions, grade-eight science curriculum, research on students' understanding of the subject, and primary school science teachers' views were taken into account. A diagnostic MCQ test was developed and administered to 108 students to determine the extent of the misconceptions revealed from the students' responses in the interviews. From the data, it was found that primary school students' misconceptions on sinking and floating, determined by earlier studies, were also held by grade-eight students in this study. In this paper, we have discussed the reasons that may lead to misconceptions and what should be done as science educators or teachers. Moreover, a sample activity, designed to remediate the misconception that mass or weight determines whether an object sinks or floats, was presented for researchers and teachers' use at the end of the paper.