The copper outer surface of the pan serves primarily to ensure exceptionally fast and even heat conduction. Copper as a material is one of the best heat conductors used in cookware manufacturing. In practice, this means that the pan heats up very quickly from the centre to the edges when placed on a heat source. This eliminates so-called 'hot spots' where food could burn while other areas are still undercooked. Just as quickly as copper heats up, it can also give off heat. This gives the chef precise and almost instant control over the cooking temperature, which is useful when making sauces or sautéing delicate ingredients, for example. The inner surface of such pans is usually made of a non-reactive material such as stainless steel or tin, as copper itself can react with acidic foods. The outer copper layer naturally changes colour and acquires a patina with time and use, which is its characteristic feature. The classic copper pan is not magnetic and therefore does not work on induction hobs on its own, unless it is fitted with a special bottom made of ferromagnetic material.