When you have the fire arrows, go to Windfall Island and find the sort of long stairs that will guide you to a switch that turns on the windmill. Go in the night and try to turn on the big light on the top by using the little "cars". When turned it on, a chest will appear on a little island nearby. It contains a piece of Heart. Reach it with a Deku Leaf. Then, talk with the man near the windmill, and he will be happy and give you a piece of Heart.