Hamburgers are usually a feature of fast food restaurants. The hamburgers served in major fast food establishments are usually mass-produced in factories and frozen for delivery to the site. These hamburgers are thin and of uniform thickness, differing from the traditional American hamburger prepared in homes and conventional restaurants, which is thicker and prepared by hand from ground beef. Most American hamburgers are round, but some fast-food chains, such as Wendy's, sell square-cut hamburgers. Hamburgers in fast food restaurants are usually grilled on a flat-top, but some firms, such as Burger King, use a gas flame grilling process. At conventional American restaurants, hamburgers may be ordered "rare", but normally are served medium-well or well-done for food safety reasons. Fast food restaurants do not usually offer this option.