AD and CF are skew because they will not intersect, and they are not on the same plane.
What are skew lines?
Skew lines are two or more straight lines that do not lie in the same plane and, consequently, do not intersect.
Unlike parallel lines, which lie in the same plane and maintain a constant separation, skew lines are non-parallel and never meet, regardless of how far they extend.
In three-dimensional space, skew lines exist when they are not coplanar, meaning they are not contained within any common flat plane.
Skew lines are non-parallel lines that do not lie in the same plane but also do not intersect.
Therefore, AD and CF are not in the same plane and are not parallel. They cannot intersect infinitly.
AD is on horizontal direction extending from point ends and CF is in a vertical orientation also extended at both ends infinitly