DULUTH, Minn. — The first thing that happens when you climb onto a mountain bike is that you become a little kid again.

It happens every time. You are immediately yanked back to those early years when a bike was a two-wheeled ticket to freedom. Suddenly, the bounds of your world were vastly expanded. You could throw a glove over the handlebars and ride to ball practice. Or you and your buddies could just cruise.

You could, in short, escape.

It happened again just the other day on the biking trails that snake along Amity Creek in east Duluth.

The first thing you're reminded of, despite having 21 gears at your disposal, is that Duluth lies on a serious hill. You gear down to a good spin, but your heart rate climbs right along with the bike. The path you're riding is no more than a foot wide in most places. If you're lucky, the trees and brush are cleared another few inches on each side of the path. You need good handlebar-eye coordination to make sure your bars dodge the occasional popple that lurks at trailside. If you miscalculate, you'll know it.

Meanwhile, your eyes race ahead, sending feedback to your brain about the trail ahead. Rock left, it says. Double root under the spruce. Your brain, based on experience it's recorded from previous rides, sends signals to your hands, your feet, your entire body. You lean. You ease off the brakes. You lift your rear end off the seat to smooth the passage over the roots.

This is the essential joy of mountain biking. It requires that you make hundreds, maybe thousands, of tiny decisions on every mile of trail, each designed to keep you upright and moving ahead. This constant assessing and adjusting keeps you on edge, and the edge is a very good place to be. When you operate on the fringes of your ability, where every decision matters, life becomes very satisfying.

"Momentum is your friend," says Duluth mountain biker Scott Kylander-Johnson.

Lose it, and suddenly balance is an issue. You do not want balance to be an issue when you're crossing a wooden bridge that's just 18 inches wide. You do not want balance to be an issue when you're riding the lip of a 40-foot drop-off to Amity Creek.

When you look ahead and see a patch of trail that looks particularly unfriendly — that is, beyond your riding level — you simply hop off. There is no dishonor in walking. Someday, you'll be ready for that stretch. But not today.

Sometimes, you attempt a challenge and fail.

I watch Chris White, an excellent rider, attack a double birch deadfall on the trail ahead. Look at that, I think. Whitey did it. Maybe I can do that.

As it turns out, no, I cannot. Not even on the fancy demo bike Whitey has lent me for the ride. Momentum, which is my friend, leaves me a millisecond after I clear the first deadfall. Stymied, I begin to fall, catching myself only at the last moment by yanking a foot from my clip pedal. But it was worth a try. Life is good on the edge.

For an hour and a half, 25 of us are lost in the reverie of our own rides in this handsome piece of woods. We climb and drop, struggle and strive, sweat and grin.

And, in the process, become kids again.