An activity is described wherein students observe dynamic floating and sinking behavior of plastic pieces in various liquids. The liquids and solids are all contained within a plastic bottle; the entire assembly is called a "density bottle". After completing a series of experiments that guides students to think about the relative densities of both the liquids and solids in the bottle, students are able to explain the curious floating and sinking phenomena. As a part of the activity, students construct their own bottles and are encouraged to describe to others how the density bottle works. These bottles can be constructed using inexpensive and easily obtained materials. The level of inquiry involved in the activity can be tailored to meet the particular interests and needs of students. Modifications to the density bottle, including an engaging one that uses LEGO pieces, are discussed.