Tiresias and the snakes

Tiresias was once watching two snakes copulating, and when he wounded the female he was turned into a woman; but later he saw the same snakes copulating again, and having wounded the male, he was transformed into a man.

Tiresias remained a woman for seven years, and became a man again in the eighth. It is told that when Zeus and Hera once disputed whether the pleasures of love are better enjoyed by women or by men, they referred to Tiresias for a decision on account of his knowledge of both sides of love. Tiresias then told them that

"Of ten parts a man enjoys one only, but a woman enjoys the full ten parts in her heart." [Apollodorus, Library 3.6.7]

For this utterance, they say, Hera blinded him, but Zeus bestowed on him the power of a seer.