A burr mill or burr grinder is a mill to grind hard, small food products between two revolving abrasive surfaces separated by a distance usually set by the user. When the distance is larger, the resulting ground material is coarser. When the two surfaces are set closer together, the resulting ground material is finer and smaller. Often the device includes a revolving screw that pushes the food through. It may be powered electrically or manually. Burr mills do not heat the ground product by friction as much as blade grinders, and produce particles of a uniform size determined by the separation between the grinding surfaces. Food burr mills are usually manufactured for a single purpose: coffee beans, dried peppercorns, coarse salt, spices, or poppy seeds, for example. Coffee mills are usually powered by electric motors; domestic pepper, salt, and spice mills, used to sprinkle a little seasoning on food, are usually operated manually, sometimes by a battery-powered motor.