Google the word "quicksand." Among the many images that pop up, you'll see a lot feature minimally clothed women half-submerged in viscous jungle sludge. Why? Because there's an online community of sex fetishists who have a thing for quicksand. Quicksand!

"I've heard of everything from feet to dirt to cars," says Justin Lehmiller, Ph.D., a sex educator and research psychologist at Harvard University. "Pretty much anything you can think of, someone out there probably has sexual associations attached to it."

When a person derives strong sexual arousal from some non-human object, a non-genital body part, or a bodily secretion, that's a rough definition of a fetish, Lehmiller says. Activities like role-playing and bondage are also lumped into the fetish category. "Basically, it's being aroused by something that is not arousing to the majority of people," Lehmiller adds.

Fetishes evolve with the times, studies suggest. Historical anthropologists have found Victorian men had a thing for bare ankles or knees-probably because women were supposed to keep them covered, the researchers speculate.

The most popular fetishes, now and in the past, center on body parts (feet or toes) and items associated with body parts (shoes, boots, gloves), indicates a study from the University of Bologna in Italy. Also, "Most people who have a fetish can remember a distinctive time or event where they encountered something that unexpectedly but immediately turned them," Lehmiller says.

But wherever they come from, fetishes tend to last, Lehmiller says. It's also typical for people to have multiple fetishes simultaneously, he explains. "You can develop new fetishes, but the new ones won't replace the others." He says a lot of people have interrelated fetishes, like a hot spot for feet, shoes, and stockings. But for others, there may not be an obvious connection, Lehmiller adds.

One thing that's certain: The emergence of the Internet has been a huge boon for fetishists. "It gives people a place to express their desires and find other people who may have the same interests," Lehmiller says. (Quicksand lovers, unite!) Here, four of the most popular theories on how fetishes made their way into your brain.

1. The brain-overlap theory. The areas of your noodle that control your sexual body parts and impulses are located alongside areas that control other appendages and emotions, studies show. (The brain region that manages your genitalia is nestled against the region that manages your feet.) These adjacent brain regions can engage in crosstalk, or overlapping activity, shows research from V.S. Ramachandran, Ph.D., of the University of California, San Diego. And crosstalk between the foot and genital regions may explain why foot fetishes (and other infatuations with non-sexual body parts) are so common, Ramachandran suggests.

2. The Pavlovian theory. Researchers in the 1960s showed a group of men images of naked women alongside images of boots, Lehmiller says. Eventually, the men became aroused by the images of boots all by themselves. This boot study suggests your brain is capable of forming sexual associations around random objects even if no arousal impulse was there to begin with, Lehmiller explains. So if you're exposed to something repeatedly during times when you're sexually aroused, your brain might come to link that object with sexual desire. "There's also some research showing people with higher sex drives are more likely to have uncommon sex interests," Lehmiller adds. Why? A super-charged sex drive leads to arousal in situations where sex with a partner isn't possible. And because there's no one around to get busy with, super randy people may unwittingly redirect their sexual energy toward whatever's in the immediate vicinity, Lehmiller adds.

3. The gross-out theory. "When you're in a high state of sexual arousal, your disgust impulse weakens," Lehmiller says. And so the things you'd normally find repulsive (feet, spit, feces) may not seem gross. "It's almost like a heightened state of arousal changes your perception of the world," Lehmiller adds. "And that changed perception might lead you to incorporate different things into your sexual acts." If you enjoy that new source of sexual stimulation, you may want to repeat whatever it is, he explains.