Skew lines are a pair of lines that do not intersect and are not parallel to each other. They can only exist in dimensions higher than 2D space, and they have to be non-coplanar, meaning that such lines exist in different planes. In two-dimensional space, two lines can either be intersecting or parallel to each other, so skew lines can never exist in 2D space. Skew lines can be found in many real-life situations. For example, if there is a line on a wall and a line on the ceiling that are not parallel to each other and do not intersect, then they can be skew lines as they lie in different planes.

To determine if two lines are skew, we can check if they are non-coplanar, meaning that they do not lie on the same plane. If each line in a pair of skew lines is defined by two points that it passes through, then these four points must not be coplanar, so they must be the vertices of a tetrahedron of nonzero volume. Another way to test if two lines are skew is to perform a quick check with a straightedge. Align it on one line and physically move it to the other line. If you have to twist, turn, rotate, or otherwise change the orientation of your straightedge to align with the second line, then the two lines are skew.

The shortest distance between two skew lines can be found by finding the distance between any two points on the two lines that are closest to each other.