The Australian Cattle Dog (ACD), or simply Cattle Dog, is a breed of herding dog originally developed in Australia for droving cattle over long distances across rough terrain. It responds well to structured training, particularly if it is interesting and challenging. It was originally bred to herd by biting, and is known to nip running children. It forms a strong attachment to its owners, and can be protective of them and their possessions. The Australian Cattle Dog breed standard states that it should have well-conditioned muscles, even when bred for companion or show purposes, and that its appearance should be symmetrical and balanced, with no individual part of the dog exaggerated. It should not look either delicate or cumbersome, as either characteristic limits the agility and endurance that is necessary for a working dog. The Australian Cattle Dog demands a high level of physical activity.