Explain tetrahedral and octahedral voids.

(N/A) $1$. Tetrahedral voids: When a second layer of spheres is placed over the first layer such that the spheres of the second layer occupy the depressions of the first layer,tetrahedral voids are formed. These voids are surrounded by $4$ spheres,and their centers form a regular tetrahedron.
$2$. Octahedral voids: These are formed when the triangular voids in the second layer lie above the triangular voids of the first layer and the triangular shapes of these voids do not overlap. These voids are surrounded by $6$ spheres,and their centers form a regular octahedron.