Various types of metal or synthetic substances are used for bit mouthpieces, which may determine how much a horse salivates or otherwise tolerates a bit; a horse having a moist mouth is considered more relaxed and responsive. Commonly used metals include stainless steel and nickel alloys, which generally do not rust and have a neutral effect on salivation; sweet iron, aurigan and copper, which generally tend to encourage salivation, and aluminum, which is considered drying and is discouraged as a mouthpiece metal. Synthetic mouthpieces may be made with or without internal metal cable or bar reinforcement. Rubber bits are generally thicker than metal bits, but other types of synthetics such as plastics are also used. Plastic-coated bits are often the same size as metal bits, and some are flavored.