Priestesses and priests in the sacred grove interpreted the rustling of the oak (or beech) leaves to determine the correct actions to be taken.
Olympia and Dodona.
By prophesy.
Dodona was the sanctuary of Zeus and home of the oldest oracle, mentioned by Homer in the Odyssey. Priests here interpreted sounds made by the sacred oaks.
His oracles were at Olympia and Dodona.
At Dodona in the Nortwest of Greece.
Oak, as Zeus gave his oracles from the sacred oaks of Dodona.
The great oracle at Dodon is reputedly the oldest in Greece. It was located in Epeiros. Today that region is currently divided between the region of Epirus in northwestern Greece and the counties of Gjirokastër, Vlore, and Berat in southern Albania.
He had 3 sisters and their name is Hestia,Hera and Demetre. And there you go.
Um, it's 'Zeus' and by magic.
There were lots of oracles, but the most famous was at Delphi, where in a cave among the rocks the Pythoness (a sort of medium) would go into a trance and make prophecies. It's possible that fumes from volcanic vents (most of Greece is an earthquake zone) assisted the process. The delphic oracle was famous for prophecies that could be taken either way - as in, 'if you go to war, you will destroy a great empire' - which might, of course, turn out to be your own.
Eagle - messenger and animal companion: he also transformed into this bird on occasion. Shield - Zeus was protector of people and gods. Thunderbolt - the weapon of Zeus, crafted by the Cyclops, used against Cronus. Oak Tree - Zeus gave his oracles from the sacred oaks of Dodona.
Dodona's Grove was created in 1640.