The term "whipped cream" refers to cream that has been beaten until it is light and fluffy, as by whipping with a mixer, whisk, or fork. Cream containing 30% or more fat can be mixed with air, and the resulting colloid is roughly double the volume of the original cream as air bubbles are captured into a network of fat droplets. If, however, the whipping is continued, the fat droplets will stick together destroying the colloid and forming butter; the remaining liquid is buttermilk. Confectioner's (icing) sugar is sometimes added to the colloid in order to stiffen the mixture and to reduce the risk of overwhipping. Milk resists the whipping and does not hold the air bubbles well. Light whipping cream contains 30% to <36% milkfat and holds the air bubbles when whipped. Heavy cream contains 36% or more fat. Whipped cream may be sold ready-to-use in pressurized containers; when the cream leaves the nozzle, it produces four times the volume of cream, twice the volume produced by whipping air into it.. Using this technique, it may also be prepared in reusable dispensers, similar to a seltzer siphon bottle, using inexpensive disposable cartridges. The whipped cream thus produced is unstable, however, and will return to a more or less liquid state within half an hour to one hour. Thus, the method is not suitable for decorating food that will not be immediately served. Cream is usually whipped with an electric or hand mixer, and with some difficulty a fork. Whipped cream is a popular topping or icing / frosting for several desserts such as pie, ice cream, cupcakes, and cake.