This paper describes investigative homework with apples, aiming to contribute to the primary-school students' understanding of density and conditions leading to floating and sinking. The assignment represents an opportunity for individual autonomous learning of physics and adoption of established scientific concepts through practical activities with easy-to-find objects. We describe the assignment and typical approaches taken by the students, followed by the detailed analysis of homework quality as a function of previous knowledge and mode of assignment (mandatory or voluntary). The research was performed on a sample of 46 primary-school students (7th grade) during preliminary testing of effects produced by active learning methods introduced into the teaching of buoyancy and related topics.