My source is the book, ‘Ancient Civilizations’, published 2006 by McGraw Hill and National Geographic.
The ancient Greeks believed that oracles were able to give prophecies, or predictions of the future. An oracle was a priest or priestess who claimed to receive advice or prophecies from the gods.
The most famous was the oracle of the Temple of Apollo at Delphi. The oracle chamber was deep inside the temple, and the room had an opening in the floor where volcanic smoke hissed from a crack in the earth. A priestess sat and listened to questions and the priests translated her answers.
The priestess often gave her answers in riddles. When one king, named Croesus, sent messengers to the Oracle at Delphi, they asked if the king should go to war with the Persians. The oracle replied that if Croesus attacked the Persians, he would destroy a mighty empire. Overjoyed to hear these words, Croesus declared war on the Persians, but the Persian army crushed his army. The mighty empire that King Croesus destroyed was his own!