According to multiple sources, including Reynolds Kitchen and the Family Handyman, both sides of aluminum foil are equally effective at heating food, and the difference in appearance between the two sides is due to the manufacturing process. The shiny side is the side milled without being in contact with another sheet of metal, while the dull side is the side in contact with another layer during the milling process. Therefore, it does not matter which side of the foil is used when cooking or storing food, except for non-stick foil. Non-stick foil has a protective coating on one side, so the dull side should be used to ensure maximum efficiency. The non-stick side is usually labeled as such on the foil.