One of Zeus' many conquests was a nymph named Io. Hera discovered his ploy and as she happened upon them pair, Zeus changed the nymph into a cow. Hera, knowing that her husband would return to his desires as soon as she departed asked him for the cow as a gift. Knowing that refusing Hera would give him away, Zeus begrudgingly did so.
Hera immediately set her favorite servant, Argus Panoptes, a giant with 100 pairs of eyes, to watch Io. Argus was the perfect watchman because he only needed to close half of his eyes in sleep at any given time. Knowing that he would never be able to sneak Io away, Zeus sent Hermes, the most cunning of the gods, to free her. Taking some wine from Dionysus, he set off to find the giant and his ward.
Meeting up with Argus, Hermes offered him some wine and soon, the giant was asking the god to tell a story (for which Hermes was a spectacualrt story teller). Hermes chose the longest and most boring story he coudl muster, all the while, filling Argus' glass over and over again. Eventually, all of the giants eyes were closed in a deep sleep, at which time Hermes slayed him gently.
When Hera discovered what had happened, she gathered Argus eyes and attached them to the tail of her favorite bird, the peacock, whose children have ever since worn them.