wht is unit cell nd how an atom at corner is shared by 8 unit cells>>?

Unit cell may be defined as smallest unit of a crystal which repeat itself to form large giant crystal structure. In 3D arrangement in single layer, corner is shared between 4 unit cell while in combined 3D the second layer is introduced which have four unit cell to share the corner of lower layer. In This way the corner of lower layer is shared by 8 unit cells.