How did Euclid come up with parallel lines?

Euclid introduced the concept of parallel lines in his work "Elements," where he defined parallel lines as lines in the same plane that do not intersect, regardless of how far they are extended. His systematic approach to geometry involved postulating basic axioms, one of which states that through a point not on a line, there is exactly one line parallel to the given line. This foundational idea laid the groundwork for Euclidean geometry and influenced subsequent mathematical thought on the nature of space and lines.