As both classroom teaching practice and literature show, many students have difficulties learning science concepts such as density. Here are some investigations that identify the relationship between density and floating through experimenting with successive dilution of a liquid, or the systematic change of concentration of a saltwater solution. Research has suggested that one important aspect of science instruction is to create an environment in which students can construct their own understanding of science concepts through active engagement. The lessons included will adopt a constructivist teaching model developed by the National Center for Improving Science Education--invitation, exploration, proposing explanations, and taking action (Raizen, et al. 1995).