Zeus had many flings with many mortals and nymphs. One such fling was with Metis. When she became full with child, Zeus became concerned, even more so when an oracle came to Zeus and told him that since he had defied his father, so would his child defy him. Terrified that this prophesy would come true, Zeus rushed to Olympus, and swallowed Metis whole.
Now Zeus had just started to relax in his throne, confident that he had just ensured that he would stay King of the Gods forever, when his head started to hurt. He drank some wine, a cure that had always helped before, but the headache just grew worse and worse. Finally, it got so that he called for Hepheastus to split his head open to relieve the pain.
Hepheastus took careful aim and then -bang-, slammed his hammer down on Zeus' head. Zeus' skull cracked wide open, and from the crack sprang a fully armed woman with steely gray eyes. So was the birth of Athena. She became the goddess of wisdom, as well as strategic warfare, and the protector of Athens.
Zeus was a mighty god, but he was also very much human.