Visit the Post Office in West Clock Town while The Postman is inside. You'll find him doing his Mental Training, and he'll challenge you to mini-game in which you have to press the A button after exactly ten seconds. Normally, the clock overhead disappears after three seconds, forcing you to guess the correct time. If you equip the Bunny Hood before speaking to The Postman, however, the clock will stay onscreen the entire time. That makes beating the Mental Training far easier.