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. Small dogs such as Chihuahuas were also used as living heating pads during illness or injury. Some believe this practice is where the idea of pain being transferred to animals from humans originated, which gave way to rituals such as burning the deceased with live dogs, such as the Techichi, to exonerate the deceased human's sins. Chihuahuas as we know them today remained a rarity until the early 20th century; the American Kennel Club (AKC) did not register a Chihuahua until 1904. A clause stating. 'if two dogs are equally good in type, the more diminutive one is preferred' was removed in 2009. Chihuahuas can be easily frightened or provoked to attack, so are generally unsuitable for homes with small children.