Twelve Symbolic Animals
It is traditional in China, when a person is born, that one animal (shuxiang) is used to symbolize this year. Shuxiang, also called shengxiao (any of the 12 animals representing the Earthly Branches), is a traditional way in China to number the years and to record a person's age.
Under the Gregorian calendar commonly used in the world, the year can be expressed by the progression since the start of the Christian era (e.g. 2002 AD). But the Chinese lunar calendar numbers the years in a different way. Ancient Chinese people invented the method to designate the years by the Heavenly Stems and Earthly Branches. The Heavenly Stems consist of ten words: jia (Heavenly Stem One), yi (Heavenly Stem Two), bing (Heavenly Stem Three), ding (Heavenly Stem Four), wu (Heavenly Stem Five), ji (Heavenly Stem Six), geng (Heavenly Stem Seven), xin (Heavenly Stem Eight), ren (Heavenly Stem Nine), and gui (Heavenly Stem Ten). And the Earthly Branches are composed of 12 words: zi (Earthly Branch One), chou (Earthly Branch Two), yin (Earthly Branch Three), mao (Earthly Branch Four), chen (Earthly Branch Five), si (Earthly Branch Six), wu (Earthly Branch Seven), wei (Earthly Branch Eight), shen (Earthly Branch Nine), you (Earthly Branch Ten), xu (Earthly Branch Eleven), and hai (Earthly Branch Twelve). Combining each of the 10 Heavenly Stems with one of the 12 Earthly Branches in sequence creates 60 chronological symbols. For example, jiazi (Heavenly Stem One Earthly Branch One), yichou (Heavenly Stem Two Earthly Branch Two), bingyin (Heavenly Stem Three Earthly Branch Three), etc. These 60 symbols are used in circles and thus each year has a chronological symbol. For example, 2001 corresponds to xinsi, 2002 to renwu in the lunar calendar. Later, people used 12 animals (rat, ox, tiger, rabbit, dragon, snake, horse, sheep, monkey, rooster, dog and pig) to correspond to the 12 Earthly Branches, forming the 12 Symbolic Animals, namely, Earthly Branch One - Rat, Earthly Branch Two - Ox, Earthly Branch Three - Tiger, Earthly Branch Four - Rabbit, Earthly Branch Five - Dragon, Earthly Branch Six - Snake, Earthly Branch Seven - Horse, Earthly Branch Eight - Sheep, Earthly Branch Nine - Monkey, Earthly Branch Ten - Rooster, Earthly Branch Eleven - Dog, and Earthly Branch Twelve - Pig. Thus the zi Year is the Year of the Rat, and the chou Year is the Year of the Ox, and the yin Year is the Year of the Tiger, etc. Therefore, when a person is born, he has an animal as his symbolic animal. The year 2002 was a renwu year under lunar calendar, also the Year of Horse, and so children born in this year are all Horse babies.
Even though Chinese people now number the years and their age under the Gregorian calendar, they still continue to use the symbolic animals. As long as people know a person's probable age and his symbolic animal, people can infer his exact age and year of birth.