Athena saw Medusa and Poseidon having intercourse. As a punishment for her offense, she turned Medusa into a terrible monster with snake hair and glowing eyes - a sight so terrifying that it turned every man to stone who looked directly at her. However, Perseus later managed to defeat Medusa by a trick: he did not look her directly in the eye, but only saw her reflection in his shield. He kept her severed head as a weapon against enemies.
