America's fastest-growing small towns (© Forbes)

The population of The Villages, Fla., grew 29% between 2007 and 2010. // © Forbes

The nice thing about living in a small town, the old saying goes, is that when you don't know what you're doing, you can be sure that someone else does. But that may not stay true for long in the fastest-growing small towns in America.

Retirees are swelling some communities quickly, such as The Villages, a planned retirement haven in central Florida. The population of The Villages and its surrounding suburbs ballooned 29%, to 93,420, between 2007 and 2010, according to the U.S. census, putting it at the top of our list. Around 87,000 of those residents live in the retirement community at the area's core, a spread of tidy, single-story homes clustered around two "town squares" with shops and restaurants. The retirement community's population is up from 68,000 in 2007, said Villages spokesman Gary Lester, and it's aiming to reach 110,000 residents by 2016. With the baby boom generation reaching retirement age, that looks like an attainable goal. (Bing Cube: See photos of The Villages)

To determine which smaller cities are growing fastest, we used census data to calculate the population growth rate between 2007 and 2010 for every Metropolitan and Micropolitan Statistical Area (MSA) with fewer than 100,000 people. These statistical areas are geographic entities defined by the U.S. government that usually encompass a core municipality and its suburbs. The truly smallest towns in America were excluded from our ranking, as micropolitan areas must have populations of at least 10,000. And though the data from 2007 and 2010 are not ideally comparable — the 2007 populations are official estimates based on the 2000 census, while the 2010 numbers are actual counts — they still provide a good snapshot of how these areas have grown.

Retirees also contributed to the rapid growth of two other places on our list: Boone, N.C., and Heber, Utah, both popular leisure destinations, coming in at No. 4 and 5. The population of the MSA including Heber, a mountain town that's a magnet for skiers and fly fishers, rose 14.6%, to 23,530, from 2007 to 2010. Its unemployment rate was just 6.6% in November and it has a median income of $65,204.

The population of Boone, a ski area nestled in the Blue Ridge Mountains, rose 14.7%, to 51,079. It has a median income of $31,967 and an unemployment rate of 7.2%, which is below the national average. It was recently named one of the 10 best places to retire by U.S. News & World Report. In addition, it's home to Appalachian State University.

"Boone is a university town, but more than most university towns it is dominated by the university," says Todd Cherry, director of the Center for Economic Research and Policy Analysis at Appalachian State. "Indeed, enrollment exceeds the town's population. Between 2007 and 2010, university enrollment increased by nearly 2,000 students, and (university) employment increased by about 200 people. That's a primary factor in Boone's recent growth."

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The West Texas community of Pecos comes in at No. 2 on our list with a 23.2% increase in population, to 13,783. Some of those people aren't there voluntarily: The MSA is home to the Reeves County Detention Complex, a privately run prison with a capacity of 3,760. In 2007, the complex entered into a contract with the Federal Bureau of Prisons to house up to 2,407 criminal aliens for at least four years.

Another factor in Pecos' expansion: a boom in oil and gas drilling enabled by fracking.

"We've had 39 drilling rigs running at a time in the county, with motels running at 100% occupancy — and four new hotels built in three years," says Bill Oglesby, head of the Pecos Economic Development Corp. Temporary residents who come for work sometimes bunk at hotels for months at a time, he says, which may have contributed to higher population counts.

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Expanded oil drilling is also driving the economy in North Dakota — especially in small towns such as Williston, which comes in at No. 6 on our list, where the population jumped by 14.6%, to 22,398. The area's unemployment rate is a shockingly low 0.9%, with a median income of $55,396.

In third place on our list: the military town of Fort Leonard Wood, Mo., where the population rose 17.9%, to 52,274. It's home to the U.S. Army Maneuver Support Center of Excellence, the U.S. Army Military Police School and other deployable units, as well as the 4th Maneuver Enhancement Brigade. That alone could explain the growth, says military spokeswoman Tiffany Wood, as it has brought "several thousand permanent … soldiers," plus their families, to the area.