Microchips are responsible for thousands of happy reunions between owners and pets every year. With the large amount of lost pets taken to shelters, microchips could also keep your pet from being euthanized or sold to a new owner.

"Microchips are only about the size of a grain of rice," said Patty Hug, a veterinary technician at the College of Veterinary Medicine & Biomedical Sciences at Texas A & M University. "They are made to last the entire life of the pet, so once it's inserted, no upkeep or replacements are necessary."

Microchips are injected through a fairly large needle. Cats tend to cope with the injection easily, but dogs often need sedation during the procedure, says Hug. She says the best time to install the microchip is while the pet is being spayed or neutered. The pet is already under anesthesia, so it won't feel any pain.

Black-and-white mug shots of former pets have begun to appear throughout the city as part of an ad campaign that aims to shed negative stereotypes of sheltered animals.

"Sheltered animals didn't do anything," proclaim the ads, which were announced Monday by Animal Care & Control of New York City, an agency that works to protect pets.

The idea behind the campaign is to stress that sheltered animals are victims of circumstance, not abuse -- and that their only crime is a lack of a home, the agency said in a statement on its Web site.