Whisky is a type of liquor made from fermented grain mash. The basic ingredients in every whisky are grain, water, and yeast. The exact mix of grains used depends on the distillers recipe and the requirements of specific categories of whisky. The grains used can include barley, wheat, corn, and rye. Single malt whisky can only be made with malted barley. Bourbon whiskey, an American-style whiskey, is made primarily from corn, and its regulations state that the spirit must be made from at least 51% corn, aged in new charred American white oak barrels. The grains are mashed and fermented to create a beer-like liquid, which is then distilled to create a clear spirit. The spirit is then aged in wooden casks, which are typically made of charred white oak, to give it its characteristic color and flavor. The aging process allows the flavor to mature, evolve, and mellow into something more complex.