A 3-dimensional object actually has THREE principle moments of inertia – the moments of inertia about the three mutually perpendicular “principle” axes. Take a rectangular book or object that has three different dimensions (length, width and height), so that it has three different moments of inertia, and try to spin it around the three principle axes (the axes that are perpendicular to each face of the object and pass through the center of it). Only one axis produces a stable spin. Which one – the one with the largest moment of inertia, the smallest moment of inertia, or the one in the middle?