One day Zeus was bored and went out onto earth where he found io a little water nymph sitting near a lake. Then Hera came down.
So Zeus turned io into a cow because Hera always got jealous when Zeus was around other women.
Then Hera asked Zeus if Hera could have the cow as a present, so Zeus sadly agreed so Hera wasn't suspicious.
Zeus knew he had to get io back and release her. When Hera found out about the cow getting away she sent a gadfly after her. (A gadfly is a horrible insect that bites cows)
The gadfly found her and io swan across the sea trying to lose it but the gadfly hung on. A sad io then travelled to Egypt. Hera thought she had suffered enough so made Zeus promise never to see io again. Then she left io in Egypt. But Egypt was a horrible place for io because she is a water nymph and there is crocodiles in the Nile... Back at Olympus, Zeus was still really bored. Surely there must be something he could do.
Picture from:http://greece.mrdonn.org/zeus.html