Uranus imprisoned Gaia's youngest children in Tartarus, deep within Earth, where they caused pain to Gaia. She shaped a great flint-bladed sickle and asked her sons to castrate Uranus. Only Cronus, youngest and most ambitious of the Titans, was willing: he ambushed his father and castrated him, casting the severed testicles into the sea. William Sale remarks that "... 'Olympus' is almost always used of [the home of the Olympian gods], but ouranos often refers to the natural sky above us without any suggestion that the gods, collectively live there". Sale concluded that the earlier seat of the gods was the actual Mount Olympus, from which the epic tradition by the time of Homer had transported them to the sky, ouranos. By the sixth century, when a "heavenly Aphrodite" (Urania) was to be distinguished from the "common Aphrodite of the people", ouranos signifies purely the celestial sphere itself. The Greek creation myth is similar to the Hurrian creation myth.