Roast duck is prepared by revolving a young duckling on a spit in an oven, according to the recipes from the Suiyuan Garden, the famous cookbook written by poet and gourmet Yuan Mei. The crisped skin of Peking duck depends on how the ducks are handled before roasting. The duck must be coated with a syrupy mixture of oil and molasses, which gives the bird its rich date-like brownish color. Air is then pumped into the duck to separate the skin from the layer of fat underneath. Boiling water is poured in through the rump, which is then plugged. The duck is then hooked and hung on the ceiling in a pre-heated, wood-burning oven (the wood of fruit trees give the duck an added special flavor) so that the birds are browned by the heat radiating from the oven walls. The duck must be turned regularly during the 40-minute cooking process. This roasting process is called menlu kaoya, or closed-door roasting, which is the traditional roasting method.