This is a piece of lycra stretched over a wooden frame (~2 sq. ft.) with an inked grid. A heavy ball will cause a localized stretching of the lycra to provide an analogy to the curvature of space caused by a massive body. Place one or more heavy balls as desired. A very light ball rolled over the surface will follow a path tracing out the gravitational potential. See also "Potential Wells" under Mechanics-Kinematics in 1-3 Dim.