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. It may be leavened or unleavened. 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). Bread is one of the oldest prepared foods, dating back to the Neolithic era, and is referred to colloquially as the "staff of life". The development of leavened bread can probably also be traced to prehistoric times.