Mask of Truth

The Mask of Truth is a recurring mask in the Legend of Zelda series. This mysterious mask, passed down by the Sheikah, allows the wearer to read the minds of Gossip Stones. The mask is shaped like the emblem of the Sheikah; namely, the eye with a tear. The retrieval of this mask is not mandatory in order to complete the games.

The Legend of Zelda: Ocarina of Time
The Mask of Truth is given to Link by the Happy Mask Salesman in the Happy Mask Shop, after Link has sold the Keaton Mask, Skull Mask, Spooky Mask, and Bunny Hood, thus completing the Happy Mask side quest.

The Legend of Zelda: Majora's Mask
Link obtains the Mask of Truth after collecting all 30 Gold Skulltula Tokens in the Swamp Spider House. This mask has a new feature — it allows the wearer to read the thoughts of animals when they are picked up and held above one's head. This can be used in order to ascertain the winner of the Doggy Race, in effect allowing Link to amass substantial amounts of Rupees and a Piece of Heart. If Link picks up the dog in Clock Town while wearing this Mask it will say something about wanting to hurt that Deku Scrub.