Select an object having approximately uniform density throughout and in the shape of a rectangular solid with no two sides equal. Almost any hardcover book will do nicely.
Let the three axes between the centers of opposite pairs of faces be labeled X, Y, Z: the X-axis is the longest, the Z-axis is the shortest, and the Y-axis is intermediate.
Pick up the object in both hands along one of the axes and toss it gently into the air, twirling it about the axis. About the X-axis it will rise, fall, and twirl stably, and about the Z-axis likewise. About the Y-axis it never will, no matter how carefully you toss it. Try it if you don’t believe me. It’s quite mind-boggling to experience.
Now, come back here and explain, please.