Dogs came from wolves, a very long time ago. They belong to a family of animals called canids, along with wolves, coyotes, foxes and jackals. Dogs have lived with people for at least 14,000 years, but scientists do not agree on when or where dogs were first tamed. One idea about how dogs began is that calmer wolves hung around people's camps to eat leftover food. Russian scientists bred the tamest foxes with each other, and after many generations the foxes acted like friendly dogs.

A puppy buried with two people about 14,000 years ago had been sick for weeks, and people took care of it. Dogs have been helping people since ancient times. People have used dogs for hunting, guarding and herding. Guide dogs help people who are blind get around. Search and rescue dogs use their noses to find missing people, including after disasters. In 1925 sled dog teams carried medicine across Alaska to the town of Nome. More than 20 mushers and their dog teams passed the medicine along in the 1925 relay.

Dogs can hear very high sounds that people cannot hear. They hear sounds as high as about 45,000 hertz, while people top out near 20,000 hertz. Their sharp hearing for high sounds probably goes back to before dogs were domesticated, when their wolf ancestors hunted small animals like mice. Dogs have two kinds of color cones where people have three, so dogs see blues and yellows well and have trouble telling red from green. Chaser, a border collie, learned the names of more than 1,000 objects, mostly toys. Dogs follow a person's pointing finger to find hidden food, even puppies that were never trained to.

A dog's sense of smell is far, far stronger than a person's. Scientists have estimated that dogs smell thousands of times better than people, though the estimates vary a lot. A dog's nose has more than 100 million smell receptors, while a person's has about 6 million. The part of a dog's brain that works on smells is about 40 times bigger than the same part of a person's brain. Dogs use their noses and their eyes to learn about the world around them.

Dogs cool off mostly by panting, not by sweating the way people do. A dog has some sweat glands in its paw pads, but they do little to cool it down. Dogs also widen blood vessels in their ears and face to cool down. Dogs' flat faces, like pugs and bulldogs have, can make it hard for them to breathe. Flat-faced dogs have a harder time cooling off than other dogs. A nearly 900-dog study found flat-faced dog breeds are at very different risks of breathing trouble: boxers and Staffordshire bull terriers rarely had it, but Pekingese and Japanese chins did.

Before you pet a dog, ask its owner if it is okay. If a grown-up says you can, let the dog sniff your closed hand first. Leave a dog alone when it is eating, sleeping or chewing on a toy. If a dog you do not know comes up to you, stay still and quiet instead of running. Even a friendly family dog can snap if it is startled, so give dogs their space. Some dogs show they are scared or angry before they bite, so stay away from any dog that growls or backs away.

Words to know

• canids: dog family
• domesticated: tamed over time
• mushers: sled drivers
• ancestors: family long ago