Celia Malone has been looking forward to this for three days.
"Whoa!" she says, through an ear-to-ear grin, as volunteer Russ Henzel releases his grip on her bicycle and she coasts out over thin air.
The Phoenix 8-year-old is riding Arizona Science Center's SkyCycle, an attraction launched this summer in the air above the center's main hallway. The yellow bicycle is balanced on a 1 1/4 inch-thick cable 14 feet above the floor, similar to a circus high-wire.
"It's either a big, huge smile or a petrified look, white-knuckling it," says center spokeswoman Ashley Carter of people's reactions to climbing aboard the bike.
Visitors coast or pedal backward the length of the 90-foot cable, then pedal forward to the launch platform. Three carabiners secure riders to the bike, and a net catches occasional dropped shoes or hats - never people.
"You can't fall off because you're clipped to the bike, and the bike will never flip over or fall off the cable," says Henzel, who outfits visitors with a harness and clips them into the bike's seat before pushing them off into space.
Though it may feel wobbly, the bike is stable, he says. A 275-pound weight hangs from its frame, offsetting the weight of the rider and bringing the contraption's center of gravity somewhere just below the bike's seat. It's a perfect illustration of counterbalancing, when one force checks an opposing force to achieve balance.
"It feels like the first time I rode a two-wheeler, when I was just learning to ride a bike," says Alyssa Bruce, a 16-year-old visiting from Modesto, Calif. She advises not looking down while you're on the bike, but Carter says seeing strangers clapping and cheering on the floor below is part of what's so exciting about the ride.
Celia, who had seen the SkyCycle on an earlier visit but hadn't had time to ride it before closing time, says the ride is "awesome."
"It felt scary, but then, when I was out there, it was like a new world to me. It felt like a flying bike."
The SkyCycle is a permanent addition to the Arizona Science Center. There is an additional charge to ride it, and cyclists must meet age, height and weight requirements. See the "Know before you go" box on this page for more information.