In a heavy saucepan, combine sugar and cornstarch. Gradually whisk in lemon juice and water. Bring to a boil. Reduce heat and cook stirring constantly until sauce is thickened and clear, about 2-3 minutes. Remove from heat and stir in lemon rind. (For orange sauce, use 1 1/2 cups orange juice in place of the lemon juice and water, and replace the lemon rind with orange rind) 0 grams of fat.