You know the drill: you're forwarded an email from an unknown source, telling you eating chicken makes your breasts bigger. You forward to all in your address book. They forward to everyone in their address book. Then the rumour's everywhere, like Chinese whispers on steroids. It's hard to know what's true and what's hooey these days, so WH asked our experts to uncover the truth behind urban myths, old wives' tales and other weird stuff.

Is it true that killed-off brain cells don't regenerate?

Yes and no. The old school of thought: you are born with a full complement of brain cells that die as you age, explains Professor Robert Vink, chair of neurosurgical research at the University of Adelaide. The new: "There are pockets of the brain - such as the hippocampus, which controls memory - that can turn over nerve (brain) cells," he says. But not all areas of the noggin can regenerate cells - including the control centre.Any toxin (plonk, pills, pot) can kill off brain cells with ease. So you still have to protect against degeneration: watch your intake of toxins, brain-train (eg, crosswords, Sudoku) and do physical exercise. "If you don't use it, you lose it," adds Professor Vink.

Does knuckle-cracking cause arthritis?

Mostly BS, says Arthritis NZ, although any damage to joints could contribute to arthritis later on. In a study of 300 people aged over 45 published in Annals of the Rheumatic Diseases, knuckle-crackers were no more likely to develop arthritis than non-crackers, but more prone to hand swelling and loss of grip strength.

If I suppress a sneeze, can I damage my organs?

"This one looks so much like BS I almost dismissed it, but a little research revealed some interesting - if only theoretical - problems," says WH health expert Dr Jodie O'Sullivan. "You have a little Eustachian tube that goes between your middle ear and throat, which helps equalise pressure. Once a sneeze starts, if you try to stop it, air is forced from the throat via this tube into the middle ear." In theory, this could cause enormous pressure that could damage your ears. "Nothing's going to explode. But I wouldn't make a habit of it," advises Dr O'Sullivan.

If I don't use it, will my muscle turn to fat?

No, explains WH fitness expert Rebecca Underdown. "It is physiologically impossible for muscle to become fat as it's made up of a different type of cell. If muscles aren't used they get smaller - but you'll still have the same number of muscle cells." Unused kilojoules are stored as fat, and smaller muscles bring your metabolic rate down. So if you take in the same amount of energy when you're not exercising as when you were working harder than a Russian gymnast, you'll gain weight (fat).

Does switching a light on and off use up more electricity than just leaving it on?

Yes, when fluorescent lamps were first invented, but times have changed. Today, there is still a small surge in power when modern lights are flicked on, but it's nothing compared to the energy racked up by leaving them on. "We can reduce carbon emissions (and power bills) by switching off lights and appliances when not in use," says WH green living expert Sarah Sheeran. "Buy eco-friendly compact fluorescent globes (CFLs)," she recommends. While CFLs may cost more, they're cheaper in the long run: they last from 6000 to 15,000 hours, versus 1000 to 2500 hours for an incandescent bulb, reports Choice magazine.

If you step on a cockroach, does it lay eggs?

We thought this rumour was the weirdest since "Marilyn Manson was Paul from The Wonder Years". But it's partly true, the same way a pregnant dog could go into premature labour if hit by a car. Cockroaches carry fertilised eggs in a case on their back until they can find somewhere to hide it, so the pressure of a shoe could literally squeeze them out, explains naturalist Martin Robinson. But there's little chance the baby roaches would survive in that situation. Shame.

Is non-organic chicken pumped full of hormones?

Falser than air-kissing. "The use of hormones in poultry was banned decades ago, and the poultry association strictly regulates this," says WH nutrition expert Karen Dickinson. According to the Poultry Industry Association of NZ, there are absolutely no growth hormones used - and they keep a close eye on the industry to ensure this remains the case. Apparently, chooks are bigger now "thanks to specialist breeders overseas, who have selectively bred birds to grow more quickly and efficiently", rather than drugs, says their website. Antibiotics are given to chickens, but first they have to be registered by the NZ Food Safety Authority to ensure safety. So, why the rumours that chicken meat beefs up your boobs? Possibly thanks to a TV show in the 1980s in which hormonal abnormalities in women in Puerto Rico were linked to feeding of oestrogen to chickens.

Can semen act as an antidepressant?

Apparently. In a study published in the Archives of Sexual Behaviour, a researcher at the State University of New York found that the more a woman was exposed to semen through sex without a condom, the less likely she was to be depressed. (How did he get funding for that?) But before you toss the rubbers, remember you'll probably be more depressed when you contract an STI, says Dr Christine Roke, national medical adviser of Family Planning. "If you're at all concerned about an unwanted pregnancy or STIs, then it's best not to risk it," says Dr Roke. WH stress less expert Dr Mary Grogan agrees. "There are evidence-based treatments for depression that don't come with the same risks. Cognitive behavioural therapy is the most researched psychological therapy for depression."

Could I cause an explosion by using my mobile at a service station?

It's unlikely. In the history of mobile phones, it has never happened. A University of Kent, UK, study found that of 243 petrol-station fires, not one was caused by a handset. Dr Geraint Emrys, chief adviser of occupational health for the Department of Labour, explains: "Earlier mobile phones used larger power packs, and the risk may have been considered more real." The risk posed by modern mobiles seems to be much less, but warning signs provide a blanket cover.

Is it true I should buy two pairs of running shoes and alternate them?

WH put this to Olympic triathlete Emma Snowsill. "I will alternate between up to four pairs; even if I run twice in one day, I won't wear the same pair," says Snowsill, who runs 80km a week. "The cushioning in shoes needs time to bounce back." For mere mortals? "Buy two pairs and alternate them, rather than wear one pair to death and risk injury," she says.

My boyfriend's cheated in the past. Will he cheat on me?

You know the cliché: "A leopard never changes his spots". True? Not necessarily. We quizzed WH sex and relationships expert Sara Chatwin. "There are some men (and women) who are destined to repeat the same patterns over and over. But more often than not different situations, different partners and age may all change someone's behaviour. Get the details of when and why he cheated... A man who was a 'love cheat' at a young age, or who had low self-esteem, may very well have reached a stage where one woman is his focus in life."

If you pluck a grey hair, will three more grow in its place?

This rumour's so old Dan Brown should write about it. "It's a myth,' says Holistic Hair trichologist Nigel Russell. Your body can't create new hair follicles, so plucking one grey hair won't generate three new ones. A trick of nature may make you think that more appear, Russell says. "Your hair goes grey in a progression, so when you pluck one grey hair out, the hairs immediately around it are probably in the process of turning grey too." Hence the belief three new whities have sprouted as karmic punishment.

If you break a bone, doesn't it heal stronger?

Nope, says Gary Hooper, associate professor of orthopaedic surgery at the University of Otago. Bones take between six weeks and three months to heal, but you don't get a bionic arm. "Generally bones will heal to the strength they were before the fracture," says Hooper. No more, no less. But bones are living material and will respond to "loading". "The more you use it, or load it, by exercising, the stronger it will get," he explains. If you do less exercise, your bones will become more brittle. "The best way to improve your bone strength after a fracture is to have a good diet, with plenty of calcium, protein and minerals, and to exercise," says Hooper. Don't ignore cardio either: University of Missouri, US, researchers found that high-impact activities, such as running, might have a greater positive effect on bone density than resistance training.

Should I remove my amalgam fillings before having a baby?

Mercury - a key ingredient in amalgam fillings - is as popular as swine right now, having been linked to behavioural problems and Alzheimer's disease. Despite a small amount of mercury leakage, it may be worse to rip old fillings out than leave them in, says dentist Dr Anthony Goswell. The NZ Ministry of Health agrees: "There is no evidence of harm to babies from treatment with dental amalgam during pregnancy; women should avoid any unnecessary medical interventions during pregnancy."

Is it true it takes more kilojoules to digest celery than are in the celery itself?

We wish. WH weight-loss expert Sharon Carey explains that celery is simply a very low-energy food. "A 20g stick provides just 10 kJ, 0.2g protein, 0.04g fat, and 0.3g carbohydrate. One stick could fuel an average woman for around two minutes, and it would probably take longer to eat the celery, so it's possible it takes more energy to burn than what it provides us with. But it's probably around what you'd be burning if you were eating nothing." So don't skip the gym in favour of celery any time soon

If I drop food, is it really OK if I pick it up within five seconds?

Like Spandau Ballet's come-back, this is something you really want to believe in. Well, you can. (Sort of.) Lydia Buchtmann, spokesperson for Food Standards Australia New Zealand, explains: "Bacteria can grow on food pretty rapidly, but if you're in the middle of cooking and you pick up a dropped ingredient, rinse it and cook it straight away, you should kill the bacteria." If it's an ingredient not to be cooked, Buchtmann adds, like for a salad or dessert, bin it. In a study at the University of Illinois, US - which won its author an Ig Nobel Prize from the Annals of Improbable Research - gummy bears and cookies were left on tiles contaminated with E. coli for five seconds. They tested positive for E. coli. Follow this rule at your own risk.

Will wearing glasses or contacts make my eyes worse?

This is a myth - there's no scientific proof that glasses can physically change an adult's eyes. If you're long-sighted (you have trouble seeing things close-up), it's because your eyeballs are literally too small. In short-sighted people (long-distance vision is tough), your peepers are too big. It's genetic: if you can inherit a big nose, you can inherit big eyeballs, says optometrist Shirley Loh. "You can get used to the blur, and if you've never had glasses you've nothing to compare it to. As soon as you don glasses and compare it to something better, you know what you've been missing," she explains. Specs simply bend light rays into focus. But if they don't worsen eyesight, why would anyone ever need a new prescription? Computers strain your eyes, says Loh. According to surveys by the American Optometrist Association, 70 to 75 per cent of computer users have vision problems. Embrace your specs, four-eyes.

Can menstrual blood attract sharks?

This is right up there with "You can't get pregnant the first time you have sex". The internet rumour: sharks can detect a single, 1ml drop of blood in an Olympic-sized swimming pool containing about one megalitre (or 1000 million drops) of water. Righto. Andrew Fox, director of the Fox Shark Research Foundation, says he hears this question constantly on shark-diving expeditions. But sharks aren't especially excited by human blood. "Plus, the scent doesn't get carried very far at all; if the tide is going at one kilometre per hour, then a shark swimming one kilometre away might smell the blood in an hour - and even then, the current would have to be unbroken," Fox says. "When we do expeditions, we throw a lot of burley or "chum" (bloody, minced-up tuna) into the water. That's what attracts sharks." Not the painters.

Are websites starting with "https" more secure than those that start with "http"?

Yep, says Anthony Hill, technical director for IT support company Geeks 2U. URLs with "http" (hyper-text transmission control) are standard, but "https" means data going between your computer and the web server is encrypted. Put simply, "s" stands for secure. If you're punching in credit card details, make sure it's on an "https" site. Also, look for a security symbol (like a lock).

How does putting a spoon in a bottle of sparkling wine keep it good?

Um, it doesn't. There's as much truth to this one as the tabloid gossip columns, says Gus Lander, a spokesperson for the Wine Society: "I'm not sure why people do it, because it doesn't work." Sparkling wines, Lander explains, release a carbon dioxide gas when opened; this gas sits on top of the wine and helps prevent oxygen getting into the wine (oxidisation). "The more wine you take out of any wine bottle, the less time you have to drink it. Wine stoppers will keep vino good for three to four days, but no more. You can also buy half-bottles if you're on your own," he says.

Is hanging upside down bad for me?

Hanging upside down is usually safe; it may be good for you, says WH yoga expert Suzanne Carson. "[It] allows blood and lymph, which can pool in the legs, to recirculate through the body. The brain and its major glands (pituitary, pineal, hypothalamus) are flushed with blood, improving function." But, she warns, don't go 180 if you're pregnant or obese, or have a detached retina, hernia, ear infection, heart condition, hypertension or recent broken bones.

Does drinking while on antibiotics make you drunker?

No, says WH health expert Dr O'Sullivan. "By telling patients to avoid alcohol, we keep them away from the pub so they can get healthy again! No antibiotics will make you more drunk. There is, however, one particular class of antibiotics that, if you drink within 24 hours of taking them, will make you vomit. This is the Flagyle group, typically prescribed for anaerobic infections."