Using a lunge rein is a common way to break in of initially train horses in English type equitation. The trainer stands with the horse on a long (lunge) rein, and works the horse mainly in circles around him or her. At first the youngster is led with the trainer, and maybe an assistant, close to it, teaching it basic commands such as walk, trot, canter, and whoa for stop. Gradually, the trainer moves farther and farther away from the horse as it becomes used to the various commands. Lungeing, as it is called, teaches the horse to obey the voice and is a useful method of early training and fittening. It should not be overdone however, as working in circles can be very hard work, particularly for young animals. The horse's first line of defense against danger, usually a predator, is fight. They will run in a scattered bunch when frighted. They will normally only turn and fight when they have no escape route of must stand their ground such as when a mare defends her foal or a stallion his harem. When traveling from one grazing ground to another, they often go more or less in single file at a walk or canter. Feral horses don't seem to use the trot gait very often. Horses live in herds as a form of defense. A lone animal is much more vulnerable to predators. To calculate the age of a horse in human terms, one year is usually thought of as four horse years. However horses have a short adolescence, a long, active adulthood, and a short decline into old age. Domestic horses which are well cared for can live a long time. However, it is common for domesticated horses to be put down when they are no longer able to work or when they fall victim to diseases, usually between the age of 20 and 25. With good care and fair work, horses can live useful, contented lives well into their twenties and even beyond.