Keaton Mask

You can find Keaton's Mask in both Ocarina of Time and Majora's Mask. In Ocarina of Time you can get it by borrowing it from The Happy Mask Shop after you have given Zelda's Letter to the guard. You can then sell it to the guard which you gave Zelda's letter to. In Majora's Mask you get it by (after 4PM on Day 3) going to The Curiosity Shop Owner in Kafei's place after delivering Anju's letter. Then if you wear the mask while cutting moving grass, like that found in North Clocktown, a Keaton will appear and ask you some questions. If you answer them all correctly, you will get a Piece of Heart.