In Norse* mythology, the giant Mimir was considered the wisest member of the group of gods known as the Aesir. He served as the guardian of Mimisbrunnr, the well of knowledge located at the base of the world tree Yggdrasill.
During the war between the Aesir and another group of gods called the Vanir, the Vanir took Mimir and a companion named Hoenir as hostages. Hoenir was treated as a chieftain by the Vanir, but without the wise Mimir he could not speak well. The Vanir felt cheated, therefore, and cut off Mimir's head and sent it back to Odin, the father of the gods, who kept it alive in a shrine near the base of Yggdrasill.
The well of knowledge sprang from the spot where Mimir's head was kept. Seeking wisdom, Odin rode to the well to drink its waters. However, Mimir allowed him to do so only after Odin left one of his eyes in the well. From then on, when Odin wished to learn secrets from the well, he asked questions to Mimir's head, which gave him the answers.