NOTES
Since we're talking about rotation, let's look at Euler's equations for rotation. These are in what is called the "body frame" meaning that the rotations, torques, and moments of inertia are all fixed to the rigid motion that is rotating. I1, I2 and I3 are the moments of inertia around the body's three principal axes. If there are no torques, then it's pretty clear that steady rotation is only possible around one of the principal axes; otherwise you'll have "tumbling" or "precession" in which the axis around which the body is rotating is constantly changing (though angular momentum is still conserved)