NEW YORK -- Visiting all 50 states is a proud achievement for a certain type of traveler, and a worthy goal.

But for those who take the quest seriously, merely crossing the border or changing planes at an airport doesn't necessarily give you the right to say you've been there.

In fact, many 50-staters have a specific litmus test for what counts -- eating a meal there, staying the night or spending a certain amount of time. Some even require what one 50-stater called a "National Geographic moment" -- a memorable experience like visiting Mount Rushmore in South Dakota or walking down Beale Street in Memphis, Tenn. -- to cross a state off your list.

Others take a more relaxed approach: Cross the border, drive through or put your feet on the ground. At Four Corners Monument, tourists often crouch on the marker where Arizona, New Mexico, Utah and Colorado intersect so they can be photographed with a hand or foot simultaneously in each of the four states. (I photographed my own children doing this on a road trip one summer.)

David Bykowski, 51, of Broken Arrow, Okla., is "on 49 with Alaska to go." He has just one regret: He spent the night in every state he's been to except for Maine, where he only had lunch. "I feel that it's cheating," he said.

Like most people who aspire to see all 50 states, Bykowski didn't start out traveling with a goal to hit all of them. Instead, he came to it accidentally after realizing he only had a few left. His job in engineering required a lot of travel and "before I knew it, we were taking the kids everywhere and seeing everything. I started counting every state we'd seen and figured out I was pretty close."

The sole criterion for counting states toward membership in a group called the All Fifty Club (www.allfiftyclub) is "that one should breathe the air and set foot on the ground. Thus driving through the state counts if you get out once, but airport layovers do not," said club founder Alicia Rovey. But many members have their own standards: "Some do not count it unless they spend the night in that state or visit the state capital. More unique ones are sighting native birds of that state, playing a round of golf, donating blood in each state."

There's no way of knowing how many people around the country and the world have been to all 50 states; the All Fifty Club has just 80 members. Membership is $10, and associate membership is available once you hit 35 states.

Robbin Holliday, 57, of Cincinnati, visited a lot of states as a kid on family road trips. As an adult, she traveled a lot as vice president of a TV station group. One day, looking through a collection of postcards she'd sent her grandmother, she realized she'd already been to 45 states. From then on, it was just a matter of crossing off what was left.

Her last state was North Dakota, which, along with Alaska and Hawaii, frequently crops up as the final frontier for would-be 50-staters. "I joked for five years that I was saving North Dakota for my honeymoon, but I never got married, so I went on my own," she said. "I went to Theodore Roosevelt National Park. It was very interesting. They have wild horses."

Luke Anderson, an administrator at the University of Colorado-Boulder, is on a quest to play disc golf -- where you throw a Frisbee disc into elevated baskets on a course -- in every state.

"In the 16 years that I've engaged in the relatively obscure but growing sport, I've played 285 disc golf courses in 39 states," said Anderson, who chronicles his trips at www.disctrips.com. "I'd like to wrap it up by age 40 -- eight years from now. It's given me an excuse to visit lots of places, and especially the places between places -- like small towns -- that I never would have had another reason to see."

In 2009, Anderson and a friend hit six states in four days on a "meticulously planned 1,300-mile loop" through Texas, Oklahoma, Arkansas, Tennessee, Mississippi and Louisiana. He says he often daydreams "about potential road trips that would efficiently knock out several more states at once."

Mike Schechter, 36, a lawyer in Seattle with all 50 states under his belt, said what makes a visit "count" was "a subject of much debate" among his law school buddies. His conclusion: "You need to transact commerce and engage in some form of human bodily function -- sleep, eat, bathroom, etc. But airport transfers/layovers do not count."

Eric Holzberg, 47, of Scarsdale, N.Y., was only missing two states -- the Dakotas -- when he got a chance to visit Aberdeen, S.D., as part of his work in the community health center industry. So he flew to Fargo, N.D., and drove the 198 miles to Aberdeen, thereby knocking both states off his list. As he arrived, his wife emailed him graphics of a marching band, a balloon and "a duck that lays golden eggs."

Visiting all 50 states is also popular among retirees, though they often start their travels earlier in life.

"From the time we were first married, we liked to travel," said Bill Fox, of Huber Heights, Ohio, now retired from the insurance business. "We drove to Florida and back; then I had a sister who moved to Texas and another in Washington state and we'd visit. We weren't really planning on seeing all 50 necessarily, but little by little we'd go to another state we hadn't planned on just to see what it looks like."

New Yorker Dorothy Wilner, 86, decided "years ago" that seeing all 50 states would be "one of my minor goals in life." She and her husband, Morton, took different routes each time they drove to see a daughter in Seattle so they could check off more states; other times they'd tack on a few days to travel if Morton, a psychologist, had a conference somewhere. But "we couldn't just drive through or fly over or stop in the airport. We had to physically have both our feet in the state on the ground," she said.

The Wilners visited every country in Europe as well, but whether she can count the tiny republic of San Marino -- located inside Italy -- is a matter of dispute. It was snowing when they arrived, and, Dorothy Wilner decided not to get out of the car. "My husband did get out of the car," she recalled. "He put two feet on the ground and said, 'I'm not giving you credit.'"