Solution
= Solution
Choose two distinct space-diagonal directions $\mathbf u=(1,1,1)$ and $\mathbf v=(-1,1,1)$. Their <dot product> is one and their <norms> are both $\sqrt3$. Thus the acute <angle> between their unoriented lines is
$$
\boxed{\theta=\arccos\frac13\simeq70.53^\circ.}
$$
Any two distinct space diagonals have an absolute <dot product> of one when represented with components $\pm1$, so they give the same acute <angle>.