Cake is a form of food that is usually sweet and often baked. Cakes normally combine some kind of flour, a sweetening agent (commonly sugar), a binding agent (generally egg, though gluten or starch are often used by vegetarians and vegans), fats (usually butter, shortening, or margarine, although a fruit puree can be substituted to avoid using fat), a liquid (milk, water or fruit juice), flavors and some form of leavening agent (such as yeast or baking powder), though many cakes lack these ingredients and instead rely on air bubbles in the dough to expand and cause the cake to rise. Cake is often frosted with buttercream or marzipan, and finished with piped borders and crystallized fruit.