Zeus gave birth to Athena- he had a headache and got Hephaestus to crack open his skull, as one does, and Athena jumped out, fully formed, armed, and ready to go!
Maybe it's like the stories of the women who think they have to go to the bathroom and then a baby shows up. He thought he had to blow his nose, but then a baby showed up. Or something. No matter how it happens, I'm thinking a giant head giving birth is probably pretty gross.
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