Copper (a) is the most even heat conductor. It is often used for saucepans, sauté pans, sugar-boiling pots, and casseroles. Copper-made pans should be lined with nickel, tin, or stainless steel (except for the sugar-boiling pots used in candy making, as the lined copper pot heats and cools too quickly) and be extremely heavy. Unlined copper pans should never be used to prepare acidic substances (those having high content of vinegar, wine, citrus juice, tomato juice, or sour milk, among other acids), as the meeting of chemical compounds causes a toxic reaction that can lead to serious illness. Never cool any sauce or soup in an unlined copper pot, as it will affect the color and taste and can cause the product to become toxic.