Equip a bottle to any button. Swing the bottle at a bug/fairy/fish. Before you catch the bug/fairy/fish, equip an item you don't need to the button of the bottle. The item will be transformed into a bottle, and will never return to it's original state. Do not replace items required for gameplay with a bottle, for you can not get it back. Replace items such as Masks, or the Trade Item after you complete the trade.