6. For a regular tetrahedron $S-A B C$ with side lengths equal to the base edge, if $E, F$ are the midpoints of $S C, A B$ respectively, then the angle formed by the skew lines $E F$ and $S A$ is equal to what?
6. Construct the cube $A S^{\prime} B C^{\prime}-A^{\prime} S B^{\prime} C$, then the triangular pyramid $S-A B C$ in the figure meets the conditions of the problem. Connect $S^{\prime} C^{\prime}$, then $E F / / S S^{\prime}, E F$ forms the same angle with $S A$ as $S S^{\prime}$ does with $S A$, and $\angle S^{\prime} S A=45^{\circ}$, so the angle between the skew lines $E F$ and $S A$ is $45^{\circ}$.