* Physical Characteristics: Dogs have distinct physical features that set them apart from other animals. They have a furry coat, four legs, a tail, pointed ears, and a wet nose. Their body structure and size can vary widely, but these characteristics are common to all dog breeds.
* Behavior: Dogs exhibit certain behaviors that are specific to their species. They bark, wag their tails, fetch, and often show loyalty and affection to their owners. They also have a natural instinct to chase and herd, stemming from their ancestry as wolves.
* Genetics: Dogs are classified as *Canis lupus familiaris*, a subspecies of the gray wolf. Their DNA is very similar to that of wolves, but they have undergone genetic changes through domestication.
* Scientific Classification: Scientists use a system of classification to group living things based on their shared characteristics. Dogs belong to the family *Canidae* which includes wolves, foxes, coyotes, and jackals.
In simple terms, we know a dog is a dog because they look, act, and are related to other dogs in a way that's distinct from other animals.