Bread is a staple food prepared by cooking a dough of flour and water and often, additional ingredients. Doughs are usually baked, but in some cuisines breads are steamed, fried, or baked on an unoiled skillet. Salt, fat and leavening agents such as yeast and baking soda are common ingredients, though bread may contain other ingredients, such as milk, egg, sugar, spice, fruit (such as raisins), vegetables (such as onion), nuts (such as walnuts) or seeds (such as poppy). Referred to colloquially as the "staff of life", bread has been prepared for at least 30,000 years. Fresh bread is prized for its taste, aroma, quality, appearance and texture. Retaining its freshness is important to keep it appetizing The soft, inner part of bread is known to bakers and other culinary professionals as the crumb, which is not to be confused with small bits of bread that often fall off, called crumbs. The outer hard portion of bread is called the crust.