below-zero temperatures due to the salt content. The sub-freezing temperature helps slowly freeze the edible mixture, making the ice cream. Some small manual units comprise a bowl with coolant-filled hollow walls. These have a volume of approximately one pint (500 ml). The paddle is often built into a plastic top. The mixture is poured into the frozen bowl and placed in a freezer. The paddles are hand-turned every ten minutes or so for a few hours until reaching the desired consistency and flavor. Electric machines There are four types of electric ice cream machines. Each has an electric motor that drives the bowl or the paddle to stir the mixture. The major difference between the four is how the cooling is performed. Counter-top machines Counter-top machines use a double-walled bowl with a solution between the walls (typically distilled water and urea) that freezes below . In a domestic freezer, this requires up to 24 hours before the machine is ready. Once frozen, the bowl is put into the machine, the mixture is added to the bowl, and the machine started. The paddles rotate, stirring the mixture as it gradually freezes through contact with the frozen bowl. After twenty to thirty minutes, the solution between the double walls thaws, and the ice cream freezes. This type of machine has the advantage of being relatively inexpensive; however, a pre-frozen bowl makes only one batch at a time. The bowl must be refrozen to make another batch. Multi-batches require extra bowls for the machine, which require extra freezer space. Small freezer-unit machines Small freezer-unit machines sit inside the freezer (or the freezer part of the refrigerator) and operate similar to a food processor in slow-motion. Every few seconds, the paddles stir the mixture to prevent formation of large ice crystals. When the ice cream sufficiently freezes, the paddles automatically stop rotating and lift. Since the mixture is cooled in the freezer, it takes longer to freeze than other ice cream makers, which work by placing the ice cream bowl in direct contact with the cooling element. A disadvantage is that the freezer door has to be closed over the flat cord, which is plugged into the nearest power outlet. However, some modern refrigerators have a built-in ice-cream maker as an accessory or a specialized electrical outlet for freezer-unit machines. It is not necessary to ¨pre¨-freeze this type of ice cream maker. However, some