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. 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.