How it All Fits Together

While the water expands as it warms, the blown-glass spheres' sizes remain virtually unchanged.

Consider the case where the temperature rises from 72° F to 74° F. At first the sphere is lighter than an equal volume of water. As the temperature increases, the water's density decreases. Finally, the weight per unit volume of the sphere is greater than that of the surrounding water and the sphere falls to the bottom.