The Chihuahua (/tʃɪˈwɑːwə, -wɑː, -ˈwaʊ.ə/ (listen); Spanish: chihuahueño) is the smallest breed of dog, and is named after the Mexican state of Chihuahua. In a 1520 letter, Hernan Cortés wrote that the Aztecs raised and sold the little dogs as food. Colonial records refer to small, nearly hairless dogs at the beginning of the 19th century, one of which claims 16th-century Conquistadores found them plentiful in the region later known as Chihuahua. With the proper training a Chihuahua needs, this dog can be extremely intelligent. The molera is predominant in the apple heads and is present in nearly all Chihuahua puppies. The molera fills in with age, but great care needs to be taken during the first six months until the skull is fully formed. Some moleras do not close completely and require extra care to prevent injury.