Although the word myth comes from mythos, an ancient Greek word that means 'a spoken or written story', a myth is more than a simple tale of villains, heroic deeds and damsels in distress. There is usually a message behind the story, often religious or social, and it is this message that is the difference between 'myth' and 'story'. Of course, the myth has to be a good story, because if it was uninteresting then it would not hold the listener's attention, or be remembered and repeated. It is not unusual to find many different variations of each myth; each retelling may leave out details that a previous version included, or add details not commonly found.
Once Upon A Time...
Long, long ago (a period in which many Greek Myths are set), there was Zeus, King of the Gods, who controlled lightning and ruled high up on Mount Olympus. Zeus was known to not only have taken his sister Hera as his bride and the Queen of Gods, but to have several other separate affairs with humans in the meantime. His reason was simple, if there were more humans that were part-god, there would be better, more moralistic and wise people.
It was because of this that Hera kept a close watch for him on Earth from high on Mount Olympus. Even Zeus feared his wife's wrath. One day, while watching, Hera noticed an oddly-placed cloud blocking her view of the Earth. She knew Zeus was fooling about with other women, and mortals at that!
She quickly arrived and discovered that he was alone with a cow. However, Hera saw through the disguise and knew that the cow was really a woman that Zeus had transformed into a cow1! The woman was a girl named Io, and although Hera was furious, she played along and took the cow and tied it to a tree (some sources say she took the cow into a cave) without admitting to Zeus that she knew who the cow was.
The tree the cow was tied to was closely guarded by Argus, a powerful warrior. Argus was known for having 100 eyes placed all over his body. No matter how tired he was and how heavily he slept, two of his eyes would always remain open. He was mighty and had slain Echidna, the terrifying mother of monsters such as:
The Sphinx – A creature with the body of a lion, wings, and the head of a woman that was defeated by Oedipus.
The Minotaur - a half-bull, half-man creature defeated by Theseus.
The Nemean Lion - a lion-like creature challenged and defeated by Heracles.
Cerberus - a triple-headed dog that guards the gate to Hades.
Chimera - a creature with heads of lion, goat, and snake with wings defeated by Bellepheron.
Ladon - a multi-headed dragon which guards the tree of Golden Apples.
The Hydra - a dragon whose heads become two heads when sliced.
Argus guarded the tree well, and Io stayed prisoner there for a long time. Zeus, terribly sorry for this, devised a plan with the messenger god, Hermes to free her. Hermes came to Argus to entertain him for a bit, because no matter how he tried, he had been unable to sneak up on him to knock him out from behind. Argus, bored with his job, naturally paid attention to Hermes as he did all sorts of things - including playing pan-pipes.
But Hermes then started into the longest, dullest, most boring story that ever existed or ever will exist - the story of pan-pipes. The dull story started to make Argus, the great and powerful 100-eyed warrior, actually fall asleep. One by one, every eye on his entire body shut until the two eyes in the natural area finally went down. Hermes brought his sword down and cut off Argus's head. Argus had truly died of boredom.
As Io was released, Hera sent a troop of bees after her (some sources say Hera sent a gadfly). Running, she went from Greece to Asia Minor to Egypt2. After this long trip, the bees were stopped, and Io became worshipped in Egypt for being a mysterious white calf.
Zeus made an agreement with Hera; Io could be a human as long as Zeus never, ever laid eyes upon her again. Io became a human, and several mortal relatives of Zeus were born to Io3.
An issue rose up afterwards though. Hermes was in a trial among the gods, for he was proclaimed by Hera to have murdered Argus. Hermes had an argument so persuasive that the gods simply could not go against him. The gods, major and minor, each had stones with their names written upon them. They were asked to cast the stones upon either Hera's feet if they believed Hermes was guilty or upon the feet of Hermes if they found him innocent. Hermes's argument caused him to be buried in stones by the end of the trial, for only a few gods tossed them at Hera's feet4.
As for Argus, to preserve his memory, Hera took his eyes and placed them upon a peacock. Ever since, peacocks had 'eyes' on each of their tail-feathers.