Traditional methods used to teach the concept of density that employ solid objects of different masses and volumes can be supplemented by enquiry activities in which students vary the mass-to-volume ratio of the same object to test ideas about density and flotation. A simple substance, Blu-Tack, is an ideal material to use in this case. The activity introduced in this article not only allows students to explore the effect of air on the density and flotation of an object, but also stimulates them to relate a macroscopic property such as density to the microscopic structure of matter. (Contains 6 figures.)