A dog is a four-legged mammal that humans domesticated from wolves thousands of years ago. Today, dogs are one of the most common pets in the world and come in hundreds of breeds, from tiny Chihuahuas that fit in your hand to massive Great Danes that stand as tall as a small pony.

Dogs have an extraordinary sense of smell (about 40 times better than humans) and excellent hearing, which makes them valuable working animals. Some dogs herd sheep, others guide people who are blind, and still others help police officers track missing persons or detect dangerous materials. Search and rescue dogs have saved many lives by finding people trapped after earthquakes or avalanches.

What makes dogs special is their relationship with humans. Unlike most animals, dogs seem to genuinely understand human emotions and communication. When you point at something, your dog will look where you're pointing. Dogs can learn to respond to dozens of words and commands. Many people say their dog is their best friend because of this unique bond.

The word dog can also be a verb meaning to follow someone persistently, as in “She dogged his footsteps until he agreed to help.” When something dogs you, it follows or troubles you persistently. A determined detective might dog a suspect, staying on their trail no matter where they go.