Why do corks always float, lead sinkers sink but clothes pegs both float and sink? This inquiry unit explored these questions and is suitable for upper primary and lower secondary students. The unit comprised sequenced activities culminating in the students making and explaining a working Cartesian Diver. The learning aim was to develop a causal explanation for floating in which students understood floating in terms of balanced forces. The research aim was to document student understandings of balanced forces. Data came from classroom observations and a discourse analysis of the Cartesian Diver activity. The unit explored prior knowledge of floating and sinking, introduced new phenomena demonstrating balanced/unbalanced forces and emphasised ‘working scientifically. The study found that a range of alternative conceptions survived alongside a set of scientifically acceptable explanations in different students. This indicates the presence of multiple subjectivities, deep-seated informal alternative discourses and some formal scientific discourses. Students can develop relational explanations for abstract phenomena but need time and guidance.