Ayla Goode, 2, at right, Aiden Faulkner, 3, watch a gentoo penguin swim in the Tennessee Aquarium's penguin exhibit Friday in Chattanooga, Tenn. The aquarium expects to receive its 20 millionth customer this weekend.

Eleven-year-old Andy Cooper reached his hand into water at the Tennessee Aquarium and touched a stingray.

"Petting them feels kind of squishy," he said.

Andy was one of thousands of children and adults visiting the Tennessee Aquarium, the world's largest freshwater aquarium, this week.

And sometime over the weekend -- probably today -- the aquarium expects its 20 millionth visitor.

"Just thinking about how many people enjoyed the aquarium over the years, 20 million people is mind-boggling," said Thom Benson, senior marketing and communications manager.

The staff plans to honor that customer with a prize package including merchandise from the gift shop, a one-year aquarium membership and passes for the Imax Theater and the River Gorge Explorer. He or she will get passes for an aquarium behind-the-scenes tour and a Chattanooga VIP pass to other area attractions.

Aquarium officials will express gratitude to all visitors by offering adult aquarium tickets and adult River Gorge Explorer tickets for $20 today. Visitors may add Imax tickets for $5 each.

No one envisioned 20 million customers when the aquarium started in 1992, said Charlie Arant, Tennessee Aquarium president and CEO. Not even people who anticipated its success could predict the downtown development that has come because of it.

Started in 1992 with a focus on the journey of the Tennessee River from its headwaters in the mountains through its long journey to the Gulf of Mexico, the aquarium added saltwater and featured sharks, stingrays and colorful reef fish in 2005 when it opened its newest addition, Ocean Journey.

The aquarium's direct economic impact on Chattanooga is more than $77.4 million each year, according to an independent study cited in the aquarium news release.

Bob Doak, president and CEO of the Chattanooga Convention and Visitors Bureau, said the aquarium marked the beginning of a downtown renaissance.

"As a community, we're so much better off because of the forward-thinking leaders that made that happen," he said. "We owe so much to Jack Lupton and all the others who forever changed our downtown while improving the quality of life and economic growth throughout our city."

Lupton inspired the aquarium when he made a rare public appearance in 1985, telling government leaders that he would make a gift of $20 million toward the development of the riverfront at Chattanooga.

Twenty years ago there was no Imax theater, no River Gorge Explorer, nor were there several other surrounding businesses, said Arant.

And instead of just watching animals through a glass, visitors can touch tree frogs, guinea pigs and stingrays.

Andy was among five bus loads of sixth-graders from Eblen Intermediate School in Asheville, N.C., visiting the aquarium on a school field trip. The school brings its sixth-grade class to the aquarium every year.

While he listened to a talk on how animals in the water produced, he stuck his hand in the pool again, this time to reach for a horseshoe crab.

"Every day there are magical moments," said Benson.