Why can two lines be coplanar?

Answer 1

Two lines are not always coplanar. If they intersect or are parallel, then

they're coplanar. But if they're skew, then they're not coplanar.

Example of parallel lines in space:

The top and bottom edges of a wall in your room.

Example of intersecting lines in space:

The top edge and left edge of a wall in your room.

Example of skew lines in space:

The top edge of one wall and the bottom edge of a wall next to it in your room.