"Dude, I'm stoked."

Certainly you've head the expression by now. Perhaps watching the X-Games. Maybe even a cute bank advertisement showing a soccer mom riding a wave. Once strictly the territory of Jeff Spicoli-type surfers, the word 'stoke' has become one of most popular pieces of slang ever, describing increasingly stranger facets of every day life, from a clean credit score to finding five bucks on the street. So what is this thing called "stoke?" Is it merely happiness? Or is it something more? Turns out everyone's got a little stoke inside them, but when and how it emerges is as unique as our own personalities.

You hear about certain athletes having "heart." Others "soul." The very brave have "balls." But if you want to discuss body parts that determine what gets you stoked, there's only one that matters: the brain -- particularly, reward centers like the nucleus accumbens -- and its little helper dopamine, the chemical neurotransmitter that equates to pleasure.

When something feels good, dopamine pummels your brain's reward centers, making sure your body never forgets the experience. The difference is your body likes the beating and then tries to repeat it. Food, sex, driving fast, working in soup kitchens, anything you enjoy outside your body turns to dopamine on the inside, including alcohol and drugs -- especially cocaine and crystal meth, which flood your brain with ungodly amounts of the neurotransmitter with a single line. In other words, dopamine is what keeps every junkie's face in the bag, every fatso's car in the drive-thru and every surfer in the water. (It also keeps every skater on a ramp, snowboarder on the mountain, golfer on the course, ...you get the idea.)

As I write this, dopamine dances across my synapses in expectation of the feeling of successfully completing a story. (Possibly more so, since I'm discussing surfing, a topic I enjoy.) But it's not the same dopamine rush I get when I'm actually surfing. That's because when you're being physical your motor cortex can take over, allowing your conscious mind to turn off. You're also putting yourself in a potentially dangerous situation where there's the possibility of harm if you make a mistake, adding a splash of adrenaline from your brain's natural wet bar.

"So, a surfer wants that feeling of getting in the perfect position where if you make the wrong move you're toast but if you stay engaged you're gonna ride right through," explains Dr. Richard Keefe, Director of {{{Sports}}} Psychology at Duke University. "That's the point where your mind stops trying. That's when you react and flow. And those are the mental states that make people's performance much better and it makes their appreciation of their sport much greater. I call it the 'effortless present.'"

Surfers call it stoke. And while surfers tend to think of themselves as the Dalai Lamas of Dudedom, spiritual beacons in a world filled with more pedestrian sports, they're really just a bunch of lab rats hammering the button that triggers another blast of chemical happiness.

Of course, like all drugs, there's a fine line the greatest high of your life and a potential overdose. So how do humans repeatedly find their optimum buzz? Once again, it comes down to the individual.

"People differ in their optimal level of stimulation," says Dr. Marvin Zuckerman, Professor Emeritus of Psychology at the University of Delaware. "Some people need a lot and some people need a little. We've defined the Sensation Seeking Personality as the seeking of novel, intense and complex experiences and the willingness to tolerate risk in the pursuit of these experiences or sensations."

Dr. Zuckerman may have never ridden a 50-foot wave or seen the top of Mount Everest, but he's done something equally scary: spent 35 years performing painstaking research on the personality types who do. He's developed a scale of Sensation Seekers, running from Highs (like mountain climbers and skydivers) to Middles (any number of mainstream contact sports) to Lows (chess club.)

Surfers and other 'action sports' athletes as a rule are Highs, since they receive an inordinate amount of exhilarating stimuli the nerve-rattling energy just by floating in the ocean or staring down the bunny slope. The problem is, unlike illegal drugs, which only require more and more of the same thing, "stoke" demands much higher levels of both intensity and novelty. That's why, today, you see guys on the Weather Channel braving waves 10 times their height or doing crazy backflips on a {{{300}}}-pound dirtbike. But they're not really insane. And, they're not necessarily ultra-courageous, either.

"Some call them risk-takers," Zuckerman explains. "But that's not appropriate because the risk is not the main point; that's just something they are willing to tolerate. High Sensation Seekers actually estimate activities as less risky than Lows do, and when exposed to the situation they experience less anxiety."

In other words, to Evel Knievel, jumping the Grand {{{Canyon}}} on a motorcycle truly sounds more fun than scary. Or, in Laird Hamilton's case, that six-story wave really doesn't look all that tall.

Okay, so what we always knew is true: from freebasing to base-jumping, some guys will do anything for a buzz. But what about everyone else? If we're not looking for the wildest sensation ever, what are we looking for?

"It's called the 'optimal level of arousal,'" says Zuckerman, "How to get the best kick with the least aversion."

Turns out the key -- like surfing -- is finding the perfect balance of risk and reward that sends a wave of chemicals crashing over your brain. And whether it happens in the ocean or on a soccer field, that's when you're stoked. Dude.