This interactive simulation helps the user to visualize the gravitational force that two objects exert on each other. It features two spherical objects whose masses can be changed by the user from 1-100 kg. It was designed to build understanding of how the gravitational force between two objects is affected by their masses and by the distance between them. Also included are teaching tips and lesson plans for use in high school and lower-level undergraduate physical science courses. This item is part of a larger collection of simulations developed by the Physics Education Technology project (PhET). The simulations are animated, interactive, and game-like environments in which students learn through exploration. All of the simulations are freely available from the PhET web site for incorporation into classes.