= Inertia tensor of a uniform cube
{title2=$\mathsf I_G=\frac{Ms^2}{6}\mathsf1$}
For a uniform <cube> of side $s$ and <mass> $M$, reflection <symmetry> makes central off-diagonal inertia components zero. Equal coordinate second moments are $\langle x^2\rangle=\langle y^2\rangle=\langle z^2\rangle=s^2/12$, so each diagonal component is $Ms^2/6$. Thus every central axis is a <principal axis>. A nonspherical shape can have an isotropic central <inertia tensor>: that tensor measures only quadratic moments of the <mass> distribution, not the whole shape.
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