What did Odin sacrifice to gain wisdom?

The story behind the answer

In Norse myth, Odin sacrificed one of his eyes to drink from Mimir's Well (Mimisbrunnr), trading sight for insight. The well, beneath a root of Yggdrasil, held cosmic wisdom, and Mimir demanded a steep price; Odin placed his eye in the water, gaining unmatched knowledge.

Odin, god of wisdom, poetry, and war, repeatedly sacrifices for knowledge—he later hangs himself on Yggdrasil for nine nights to discover the runes. Memory tip: think "Odin’s I for eye = insight"—he gives an eye so his "I" becomes all-seeing.