Mask of Truth

In both Ocarina of Time and Majora's Mask, when you wear the Mask of Truth and observe a Gossip Stone it will tell you a secret about the characters and world. If you pick up certain animals with this mask on, you can read their thoughts.

In Ocarina of Time, you must sell all the masks from the Happy Mask Shop in Hyrule Market. In Majora's Mask you must complete the Swamp Spider House located in Woodfall.