A double boil, also known as a bain-marie or water bath, is a cooking method in which a pot of simmering water is used to gently heat and cook another vessel placed inside it. The vessel, which can be made of glass, ceramic, or metal, is typically used to melt chocolate, cook custards, or gently heat other delicate ingredients without exposing them to direct heat. It is also useful for cooking custards or sauces that may curdle or burn if cooked directly over heat.