Once, when Tiresias was still young, he caught sight of two snakes mating near Mount Cyllene in Arcadia/ Peloponnese. Brutally, he tried to injure the snakes to make them split. All of a sudden, he saw himself transform into a woman...
Seven years later, Tiresias again saw another a couple of snakes mating and on that moment he became a man again.
Up on Mount Olympus, Zeus, the king of the gods, and his wife Hera were arguing one day as to whether men or women had more pleasure in love.
"Let's ask Tiresias from Thebes. He knows best. After all, he has been both man and woman in his life!"
So they agreed to fetch Tiresias and consult him.
"Now tell us, what is your honest opinion on that?", they asked Tiresias.
"On a scale of ten, I can say that men get one part of pleasure, whereas women get nine parts!", Tiresias replied.
For that reason, Hera turned Tiresias blind. But Zeus took pity on him and granted him the gift of prophecy and a very long life.