The Happy Mask side quest is a trading quest from The Legend of Zelda: Ocarina of Time. It begins when Link shows the soldier at the Death Mountain Trail Starting Point Princess Zelda's letter, allowing him to scale Death Mountain, and the Hyrulean Soldier asks Link to bring him a popular mask for his son. At the end of the side quest Link earns the ability to borrow the Mask of Truth, which allows him to listen to the secrets of the Gossip Stones scattered throughout Hyrule.
First, Link must go to Hyrule Castle Town and enter the Happy Mask Shop. Inside, he should speak to the Happy Mask Salesman and borrow the only mask available, the Keaton Mask. Then Link must speak to the Hylian Soldier at the Death Mountain Trail Starting Point while wearing the mask. He will give Link a certain amount of Rupees to keep and give back to the salesman.
The Happy Mask Shop now has a Skull Mask that Link can borrow after he pays the Happy Mask Salesman for the Keaton Mask. Link must go to the Lost Woods and find the open area with only one Skull Kid in it. If Link has not already befriended the Skull Kid, then he should stand on the stump, Z-target Skull Kid, and play Saria's Song. Once the Skull Kid knows Link's face, Link can put on the Skull Mask and speak to him. The Skull Kid will ask if he can have the mask and give Link 10 rupees. The text will mention it is an unfair deal, as the mask costs 20, and Link will have to pay the difference back to the Happy Mask Salesman out of his own pocket.
Link can now borrow the next mask, the Spooky Mask. Once he borrows it, Link must go to Kakariko Village during the day. He then should go to the Kakariko Graveyard, wear the mask, and talk to the Boy in the Graveyard. He will get scared thinking that Link is Dampé the gravekeeper. Once he realizes that Link is not the gravekeeper, he will ask for the mask to look more fearsome.
Link can now borrow the final mask, the Bunny Hood, from the Happy Mask Salesman. He must then go to Hyrule Field just west of Lon Lon Ranch at night once he has completed Inside Jabu-Jabu's Belly. Link should then wear the Bunny Hood and watch for the Running Man. When he sits to take a rest, Link can talk to him. He will fill up Link's Wallet completely (it is best to have the Giant's Wallet for more Rupees). Link can now go back to the Happy Mask Shop to complete the quest.
Link is now able to borrow the Mask of Truth, which allows him to listen to Gossip Stones. He can also get the Gerudo Mask, Goron Mask, and the Zora Mask, though these are only useful for getting comical reactions from people.