Judy Kei can tell when her cat, Kit-Kat, isn't feeling well. It moves around less than normal, seems lethargic and then nibbles on a planter of cat grass in her apartment. 'Occasionally, Kitty would get an upset stomach, so it would eat the grass and make itself sick,' Kei says. 'I think sometimes the food is too rich for it. And it seems better after vomiting; it's like taking natural medicine for cats.' Some cats can't seem to resist the odd blade of grass, while other felines will turn their noses up at anything green. Veterinary surgeon Kylie Griffin, at The Ark Veterinary Hospital, shares some insight into which household plants are edible, and which ones are highly toxic and harmful for your four-legged nibbler. 'Some cats like eating plants. They seem to enjoy it, investigating and smelling it,' Griffin says. She adds kittens tend to explore their environment with their mouths. Often long foreign bodies, such as string, needles and thread, are consumed out of curiosity, the veterinarian says. Most of the time, these items are vomited out or pass through the body. On rare occasions, however, a piece of string or thread can get caught in the mouth and loop around the tongue. Griffin says there isn't any scientific evidence that suggests eating grass or plants is a necessary part of a cat's diet. 'Some cats do seem to need to chew grass, but others don't chew at all and still stay nice and healthy,' she adds. Many theories try to explain why cats eat grass. Some say grass helps cats to vomit fur balls, or the fibre in grass is nutritional. As cats ingest their fur, while licking their coat, some will vomit up fur balls more frequently than others. In addition, through selective breeding, domesticated cats with 'unnatural coats' of very long or very fine hairs may have trouble digesting their fur. It's possible that grass helps the fur pass through the digestive tracts, Griffin says, but as only some cats eat grass it doesn't seem essential for survival. Research suggests cats hardly need any fibre. Some fibre can be found in meat that seems to help digestive problems. However, there are no studies that reveal cats require fibre from grass. Most young grass contains vitamin B, and numerous products of individual packets of cat grass are available, purporting grass as an essential part of their diet. Griffin adds it's obvious some cats enjoy chewing on grass and plants. Out of all the potentially harmful plants to your cat, lilies are the most dangerous. 'Number one for cats is lilies. They can become very sick and can even die from kidney or liver failure if your cat eats the flower,' Griffin explains. Cats that may have ingested some lily flower or its leaves may seem lethargic, have a loss of appetite or recently vomited. 'It won't be feeling very well. You'll probably have to go and find your cat hiding somewhere under the bed or in the wardrobe.' If you suspect lily poisoning, then take your cat to see a veterinarian. Griffin says there's no antidote, only supportive treatment, such as anti-nausea medicine and liver protectant. For severe cases, a cat might stay at the veterinarian's for a week to be observed and force-fed. Griffin adds, fortunately, most owners today know of the dangers associated with cats and lilies. Another plant that is toxic to cats is dieffenbachia, a leafy household plant that survives well outdoors in Hong Kong's tropical weather. Its light yellow and green leaves contain needle-shaped calcium oxalate crystals that can cause localised swelling and oral irritation when chewed. But the effects are usually not fatal. Ribbon plants, also known as dracaena or dragon tree, are another no-no for furry felines. A few nibbles of this tall, ribbon-like plant can cause vomiting - possibly with blood - anorexia, dilated pupils and hypersalivation. As there are spray-on products that deter cats from chewing on toxic plants, the veterinarian recommends the best method of prevention is to remove the plants or flowers from your home. Griffin says ferns and water plants are non-toxic for felines. Others include bamboo, petunias, spider ivy and tiger orchid. 'I know a cat that would go crazy when you open an avocado. Every cat is different, I'm not sure if anyone truly understands them,' she says.