This exhibit is a museum-sized version of a toy many of us made as kids, namely a cartesian diver. Two objects in a tall tube of water can be made to rise or sink by compressing or stretching a rubber membrane over the top of the tube. One of the objects normally floats when the exhibit is undisturbed and the other object is bi-stable, either floating at the surface of the liquid or resting on the bottom, where ever it is left.