A pet is a household animal kept for companionship and a person’s enjoyment. The most popular pets are noted for their loyal or playful characteristics, for their attractive appearance, and/or for their song. Pets also generally seem to provide their owners with non-trivial health benefits; keeping pets has been shown to help relieve stress to those who like having animals around. There is now a medically-approved class of “therapy animals”, mostly dogs, that are brought to visit confined humans. Walking a dog can provide both the owner and the dog with exercise, fresh air, and social interaction. The most popular pets are dogs and cats.