5. It is known that the structure of the methane molecule $\mathrm{CH}_{4}$ is: the center is a carbon atom, surrounded by 4 hydrogen atoms (these four hydrogen atoms form the four vertices of a regular tetrahedron). Let the angle between the four line segments connecting the central carbon atom to the outer 4 hydrogen atoms be $\theta$, then $\cos \theta=$ $\qquad$