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 $\cos \theta=$ $\qquad$
5. $-\frac{1}{3}$ Detailed explanation: It is easy to know that the required angle is $\angle A O B$. Let the edge length of the regular tetrahedron be 1, then $A O=\frac{\sqrt{6}}{4}=B O, A B=1$. By the cosine rule, we get $\cos \theta=-\frac{1}{3}$.