No Kitty, No Chewing!
Cats can do a surprising amount of damage by chewing. Kittens may chew when teething or just for fun, whereas adult cats are more likely to chew out of boredom or because of obsessive compulsive behaviors. And of course, cats of all ages love to chew houseplants.
Bored cats tend to get into mischief chewing all sorts of objects. One day it may be your plastic lids you neglected to put away, another day it may be your favorite hairclip, and another it may be the old photo collection you were admiring on your desk. Be sure to give your cat plenty of interactive play time, and when you can't be with him, bring out a couple of interactive cat toys that release tiny food treats when he manipulates them. Change the toys every few days so he doesn't get tired of them. Give him exercise toys that allow him to jump and bat at objects. And give him something appropriate to chew. Try dog rawhide treats that have been soaked in meat or tuna juice. This should only be given to him when you are there to supervise and can make sure he doesn't try to swallow any chunks. When you're not there, give him cubes of frozen wet cat food or tuna to chew and lick. True, cats aren't crazy about cold food, but they will often make the exception if it's tasty enough---or if they have a strong enough urge to chew.
Electric cords are one of the most dangerous things your cat can chew. Cover cords with split hollow tubing, or make the area around the cords unappealing by covering it with two-sided tape or aluminum foil, both of which are unpleasant for cats to stand on. You can also buy motion-detecting devices made just for cats. These devices emit a harmless but annoying spray and a high-pitched alarm. As for the cords, spray them with a bitter cat-deterring flavor available from your veterinarian or pet supply store. Since cats are pretty particular about taste, they are very effective. Even spray-on human deodorant or cheap perfume will work to dissuade most cats.
Houseplants are a natural enticement for cats. But they, too, can be dangerous. Some, like cacti, are dangerous just because they are pointy. Others, like maryllis, caladium, creeping Charlie, Easter lily, dieffenbachia, ivy, mistletoe, philodendron, and poinsettia can cause mild to serious vomiting, diarrhea, difficulty breathing, collapse or kidney failure. Always check your plant's possible toxicity and place dangerous plants out of reach, or better, get rid of them. For your other plants that you wish to rescue, place them out of reach or use the same precautions you did with cords: make the area around them unappealing to stand on, place motion detectors with alarms around them, or spray them with unsavory pet-deterring sprays. But don't expect your cat to go without his vegetables. Instead, give your cat a safe source of greens to chew on. You can buy pre-seeded boxes of cat-safe greenery, or make your own window box with wheat grass and catnip.
A special type of chewing seen in some cats appears to be a type of obsessive-compulsive disorder (OCD). It most often manifests itself as chewing or sucking on certain fabrics, particularly wool. Even the hair on your head seems to be a favorite! Although any cat can have this behavior, it's most often seen in the oriental breeds, such as Siamese cats and their relatives. Because of this, and because OCD has been shown to have a genetic basis in other species, a genetic component is suspected. In addition, there may be an experiential component. Cats that are weaned too early seem more likely to develop this behavior as kittens. Because oriental breeds tend to naturally suckle to an older age compared to kittens of other breeds, it's possible that the time at which breeders wean them may be too early, contributing to the condition. Many cats outgrow the fabric sucking/chewing behavior by a couple of years of age, but others retain it throughout life.
To discourage this behavior, try the same tactics you used to discourage other types of chewing: place the preferred objects out of reach, slather them with bad-tasting substances, squirt the cat with water from a squirt gun when caught in the act, and provide safer and more enticing toys and chew items to occupy your cat. If the behavior is a true OCD, however, these efforts may not work, especially if the behavior has already become habitual. In these cases you need to talk to your veterinarian about prescription drugs that combat OCD. You may also wish to consult a veterinary behaviorist, who can prescribe both behavior therapy and drug therapy. The sooner this behavior is addressed, the easier it is to fix---and the fewer new clothes and blankets you'll have to replace!
Chewing any foreign object can be dangerous if the cat swallows any of it, possibly causing an intestinal obstruction. If you notice your cat has swallowed a large piece of anything, watch for vomiting, diarrhea, lack of appetite, or lethargy and contact your veterinarian at once.