Circles, ridges, and odd-looking continents are usually found on maps, but for people with benign migratory glossitis, these cartographic elements are right on their body.

On their tongue, that is.

John Rudzinski, a 48-year-old illustrator from Ottawa has had a "geographic tongue" as long as he can remember. "It's not always the same," he says. "On the flat of the tongue, you'll sometimes see continent-like shapes, but they change. You'll have one that's red and one that's kind of whitish and then they're gone the next day."

Geographic tongue is completely benign, say experts, and there's no treatment. Still the strange-looking patterns that appear and disappear and migrate overnight can be disconcerting.

"I'll see people who are very concerned about their tongue and they'll complain that they've had it their whole life and the doctors won't treat it," says Dr. Alan Hirsch, founder and neurological director of the Smell & Taste Treatment and Research Foundation in Chicago, calling it a benign, asymptomatic condition. "They want a treatment, they want a disease." Hirsch instead tells his patients, "Don't be scared. It'll go away on its own."

According to a recent case study in the New England Journal of Medicine, these shape-shifting tongue "lesions" are caused by the atrophying of the filiform papillae — the protuberances that contain taste buds. They can be found in about 2 percent of the world's population.

But Hirsch says it may even be more prevalent than that.

"We see it all the time," he says. "I wouldn't be surprised if it affected at least a quarter of the population."

Few studies have been done on geographic tongue. However, a 1976 study in an oral health journal does suggest it's hereditary. It's also commonly mistaken for something else.

"Your doctor might say this is fungus and shoot you with an anti-fungal agent, but it's not fungus," says Hirsch. "You can also see things that look a lot like geographic tongue with vitamin B12 deficiency, but that's different. It's basically just your own little tattoo, a tattoo that shifts over time."

Not surprisingly, it's not the only tongue condition out there. Scrotal tongue, which is sometimes associated with geographic tongue, causes deep fissures to appear on the tongue, making it look wrinkled. It, too, is harmless. An overgrowth of bacteria in the mouth can also cause black, hairy tongue which basically looks just like it sounds. Aside from putting a damper on your love life for a short period of time, black, hairy tongue is temporary, benign and goes away on its own.

Rudzinski says he never thought much about the ever-changing patches of red and white on his tongue until his dentist mentioned he had a geographic tongue.

"I thought it was normal," he says. "But then my dentist pointed it out one day and started calling in other people to look at it."

Still, he doesn't mind the continental drift going on in his mouth.

"There's no down side to it," he says. "It doesn't hurt. It just looks odd. But nobody knows you have it unless you stick your tongue out at them."

What about you? Do you have geographic tongue (or some other tongue condition) that people have pointed out to you? Tell us what you think in the comments.

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Wondering what it is about Kate Bosworth's looks that are so riveting? It just might be her distinctive irises. Born with heterochromia, Bosworth has one blue eye and one eye with sectors of both hazel and blue.

It's a rare defect that many people find irresistible.

"It's definitely been my ace in the hole," says Ingrid Ingerson, a 36-year-old Seattle program assistant who also has sectoral heterochromia. She has hazel eyes with a splash of brown in her right eye. "It's always been a hit with the boys. Not that I care about that now that I'm married, but it's still part of my cachet and part of my mystique."

According to Dr. Ivan Schwab, an ophthalmology professor at UC Davis, heterochromia occurs more frequently in cats and dogs and is uncommon in humans, affecting only about 1 in 10,000 people. It's often genetic — usually due to an excess or lack of melanin in the iris — but it can also be caused by injury, inflammation, tumor or medications, such as eye drops for glaucoma or eyelash enhancement.

In some people, heterochromia produces two completely different-colored irises. Actress Jane Seymour has one green eye and one hazel eye. Others have a colored sector (like Bosworth) or will have central heterochromia, which is basically a ring of extra color between the pupil and the iris.

"Central is pretty common," says Schwab. "If you look really close in a person with blue eyes, at the margin [of the pupil] you'll see lots of yellow dots that will blend in when you step away. It's sort of the difference between a light blue and a steely blue."

Inherited heterochromia is harmless, although acquired may be a different story, says Schwab. The worst part of it is dealing with people who've never encountered it before.

Slideshow: Celebrities with unusual eyes

"Some people can't let it go," says Ingerson. "They'll say, 'What's wrong with your eye?' 'Can you see out of it?' They … want to know if it hurts. They always go to the place where it's a problem or a malady."

Other common questions – gleaned from a "People With Sectoral Heterochromia" Facebook page – include: "Is that a glass eye?"; "I think your eye is bleeding"; and "Wow, you look just like a Husky."

While constant questions can be off-putting, most people seem to like it.

"It's exotic," says Schwab, the ophthalmologist. "It's striking and it makes them different."

It might even give them a certain star power. In addition to Bosworth and Seymour, luminaries sporting some form of heterochromia include actors Kiefer Sutherland, Christopher Walken, Dan Aykroyd, and Elizabeth Berkeley; singer/songwriter Carly Simon; baseball pitcher Max Scherzer; and by some reports, Alexander the Great.

David Bowie, perhaps the most famous "heterochromiac" in existence, actually has an enlarged pupil in one eye, courtesy of a fistfight in his youth, which gives the appearance of two different-colored irises.

Are you a heterochromiac? Tell us about your multicolored eyes in the comments.

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