For thousands of years，people have lived with dogs. Ancient paintings on the walls of caves（岩洞）show people living with dogs. More than one third of the homes in the United States，Canada，and Australia have dogs. Almost one fourth of the homes in England have dogs. In each country，these dogs come in all shapes and sizes.
The largest dog in the world was Zorba，a huge mastiff（大驯犬）that died in . When Zorba was seven years old in ，he was centimeters tall. In other words，Zorba was more than half as tall as an adult man. At his largest，Zorba weighed more than kilograms.
While the smallest dog ever was a Yorkshire Terrier dog named Sylvia from England. This dog was only centimeters tall and just under centimeters from nose to tail. The dog weighed half as much as your English book，and even a young child could easily pick it up with one hand. It died in when it was only two years old.
Some dogs are unusual，not for their size，but for their brain. One unbelievable dog is Endal，the "family member" of a man named Allen Parton，who has used a wheelchair（轮椅）since a car accident in .
In 2001 ，Parton was hit by a car while crossing a road with Endal，and thrown out of his chair. Endal quickly moved Parton into the safe position，covered him with a coat，and pushed his mobile phone close enough for him to reach. Then，once he saw that Parton was all right，Endal ran back and forthbetween a nearby hotel and Parton，barking until people came out to help.
Endal won a prize for his courage and cleverness，and he has been the subject of a number of TV documentaries.
