GRANDPA said if you don't use it, you lose it. Millions of our purring pals are brain-dead; they've lost it upstairs, while at the same time they've gained it around the middle. And their owners have no idea, because that's how we expect cats to be.

It's great that more people are restricting their cats to life indoors. Cats that wander free outdoors get hit by cars (the No. I cause of death for outdoor cats), are exposed to infectious diseases and are prone to fights, with other cats, dogs and wildlife.

Veterinary behaviorist Nicholas Dodman, director of the behavior clinic at Tufts University School of Veterinary Medicine in North Grafton, Mass., concedes: "The truth is life may be more exhilarating for cats allowed outdoors. However, as a veterinarian, it's my job to encourage safety and good health. Cats should be indoors. The problem is indoor life can be downright boring."

Karen Overall, a veterinary behaviorist based in Philadelphia says life for millions of indoor cats is worse: They're "clinically depressed." These couch-potato cats robotically move from the sofa to the food dish to the litter box. And they may have given up once-tidy habits, such as self-grooming.

Estimates from scientific studies indicate anywhere from about one-quarter to half of all indoor cats are overweight. Overall notes that makes them vulnerable to various diseases. But she is equally concerned about chubby kitties' mental fitness. Although intelligence is difficult to define, experts agree cats and dogs are in about the same class.

"We'd call it abuse to allow millions of dogs to vegetate, yet as a society we're content when our cats do just that," says Pam Johnson-Bennett, author of Think Like a Cat, (PENGUIN PUTNAM, $l6.95).

A 74-year-old woman living in a Manhattan high-rise has a Chihuahua named Chiquita. The petite pooch is litter-box trained, so she hardly goes outdoors. Chiquita's owner beams with pride as the dog rolls over, barks on command and "shakes paws." Visitors would have no idea the woman has a cat, too, until she hauls it out from under the bed. "People think cats are naturally antisocial, and that’s just not true," Overall says. "It's not that we don't love our cats; it's that our expectations of cats are based on mistaken notions."

Karen Thomas, an animal trainer for Friskies, the cat food company, travels around the country with movie- and TV-star cats, demonstrating that they can be trained to open doors, wave at a crowd and even walk a high wire (well, not all that high). 'People don’t think about training cats because cats don't have to interact in the real world as dogs do," she says. "But I guarantee cats are just as trainable as dogs. You just do it differently."

But why bother training cats? "The bond you have with your cat will forever be changed. Your cat will learn how to learn, and you’ll learn that cats can learn - despite everything you’ve ever thought about cats," says biologist Karen Pryor, who is credited with pioneering the technique of clicker training for dogs. She has authored Getting Started: Clicker Training for Cats) (SUNSHINE BOOKS, $12.95).

You won't find "terminal boredom" listed as a condition in medical textbooks, but it should he there. Boredom fosters ill health. Visual and mental stimulation turns out to be just as healthful for cats as for dogs. Luckily, homes can easily be transformed into kitty playgrounds, and that makes life much more satisfying.

STEVE DALE, a correspondent for Dog World, writes a syndicated newspaper column, "My Pet World." He also hosts Pet Central at WGN Radio and Animal Planet Radio.

Cat-and-mouse games

Teaching your cat tricks can make a huge difference. Here are some ideas to get your cat off the couch and into the real world, even if that's indoors:

Get your cat a pet. Nicholas Dodman, author of "The Cat Who Cried for Help" (BANTAM BOOKS, $12.95), says his kitties perked up when his kids bought pet rats. A fish tank or even a lizard can be like cable TV for cats. Of course, the family must be committed to caring for the new animals - and protecting them from the cat so they don't become an hors d'oeuvre. Otherwise, there are lizard and bird videos made for feline viewers.

Not all cats like the same games or the same toys. And cats get bored fast, so rotate the toys.

Cats enjoy the companionship of another cat, a dog or even a large parrot. Just be sure to gradually and carefully introduce any new animal into the household.

Play with your cat. "Use an interactive toy, and pretend it's a real meal - like a bug [a Cat Dancer toy], a bird (a fishing pole toy with feathers) or a snake [a Cat Charmer toy]," says Pam Johnson-Bennett, author of "Think Like a Cat". "All cats live to be hunters."

Allow your cat outdoors in protected safety. Build your own "cat run" out of plywood and mesh, or purchase a do-it-yourself kit at a pet supply store.

Let your cat "hunt"' for her food. You can teach your cat to bat around Play-N-Treat balls, little plastic gizmos that kibble falls out of when rolled. Eventually, hide the balls around the house so your cat can put her prey drive to practical use at mealtime.

Provide a ledge with a view. Watching an outdoor bird feeder is like live theater for cats.