Our physics students like to watch a ball bouncing underwater. They do this by dropping a weighted plastic ball into a 1000-ml cylinder filled with a linearly stratified salt-water solution at room temperature. The ball oscillates and comes to rest at about mid-depth. Its motion is analogous to the damped vertical oscillations of a mass hanging from a spring. Our apparatus for producing precise linear salt gradients is simple, inexpensive (cost about $50), and is easy to use. A refractometer is used to measure salt content (or density) of samples taken at various depths. Here we specify fresh- and salt-water volumes for making density gradients suitable for a class demonstration or a three-hour laboratory class.