People often say that a dog is man's best friend. Over thousands of years, man has taught his dogs to do many kinds of work besides guarding the home. For example, sheepdogs are famous for their ability to control a flock of hundreds of sheep. Dogs have been used to aid disabled people for centuries. A guide dog can lead its blind owner. Nowadays, dogs can be taught to turn on light switches, open refrigerator doors and dial the telephone for their disabled owners. For the majority of people, however, dogs are simply pets and friends for both young and old members of the family.