The law of conservation of mass can be counterintuitive for most students because they often think the mass of a substance is related to its physical state. As a result, students may hold a number of alternative conceptions related to this concept, including, for example, the believe that gas has no mass, that solids have greater mass than fluids, or that matter (like salt) is destroyed when it dissolves (Driver et al. 1994). Given these issues, the authors developed a lesson that can be used by teachers to help students understand the law of conservation of mass and use it to make sense of new observations. (Contains 2 figures.)