Odin recounts in the Norse Edda:

I know that I hung on a high windy tree
for nine long nights;
pierced by a spear – Odin’s pledge-
given myself to myself.
No one can tell about that tree,
From what deep roots it rises.

Nine mighty songs I learned …
and I came to drink of that costly mead
the holy vessel held (Cup of Hermes, Holy Grail).

Thus I learned the secret lore,
prospered and waxed in Wisdom.