I got asked to write the introduction to a gorgeous book called The Noblest Invention, published by Rodale. I was really late with my story, and when the drop-dead date came I was lying flat on my back, in near-paralysis from some kind of nerve problem or disc damage from overtraining and idiocy. I literally wrote this lying down. People seem to like it anyway. Right after it came out, Ali Clark, who was working for the bicycle manufacturer Trek, wrote me a letter saying that the story made her realize she’d somehow lost track of the passion she’d once had for bikes. Within a year, I think, she’d broken off her engagement, quit her job, and, last I heard, was riding across Europe. If this story had even the smallest part in that I am, I’m almost ashamed to say, happy for it.

One sparkling bright summer day when I was 11, my bike and I ran away from home. I forgot long ago what childish sin I committed but I know that as punishment my bike and I, unbearably, were to be separated for some time.

I’d gotten the off-brand BMX bike as a present only that spring. It had an oval number plate, “pro-authorized” race decals and crash pads on the crossbar and top tube. The pro-style nubs on the right grip had worn off; my friends and I all rigged our right grips to twist in our hands like motorcycle throttles without sliding off the handlebar. I ran a playing card against the spokes, of course, choosing replacements according to my mood on whatever day the old paper rectangle finally shredded away. (I favored aces and jacks.)

Years later, as a racing cyclist — a “serious” cyclist — I would learn to despise kickstands for their extra weight and aura of geekiness. But on my BMXer, I showcased the kickstand by spray-painting it metallic silver like a tailpipe. The back edge of the seat was curled and crisped; a friend of mine had convinced me that by taping model-rocket engines to the underside of our saddles we could get a burst of speed upon ignition. (All we got were scorched seats, flat tires, severe reprimands from Mom and Dad — and the admiration of every kid in the neighborhood.)

It’s not unusual that I can remember every detail of my bike. Esteemed writer Samuel Beckett, describing a bike of his youth, wrote “To describe it at length would be a pleasure. It had a little red horn instead of the bell. To blow this horn was for me a real pleasure, almost a vice.”

Yet even more vivid to me than my memory of that bike is how it called to me that day. I’d been riding for about four years but felt myself newly mobile on that big-kid’s bicycle — it was never meant to accept training wheels, and it had never known me when I couldn’t ride. That was a time when we rode our bikes in packs, kin to the dogs that were also free to roam neighborhoods at will. You rode your bike, dropped it wherever a game started, rode it another block dropped it, and played more. Your bike was always there. Occasionally a group of kids would go out expressly to “ride,” but most often our bikes were not merely a childish game but our accomplices in the entire adventure of being children.

So, picture a day from your childhood, a perfect day — bright, sunny, filled with friends and bikes. Your legs kick. You squint into a headwind of your own creation and gulp at it in greedy joy, like a dog with its head stuck out of a car window. That was my day. The bike rolled with a kind of righteous inevitability; we might go anywhere. My own block disappeared under the rear wheel, then the familiar streets of my neighborhood, then the roads my parents drove. I saw a lake. There were no lakes near our home.

My bike and I stopped. I kickstanded it, left it leaning against itself, wheel turned as if it were watching me, curious.

While I played at the water’s edge, someone stole my bike.

There is no more useful toy than a bicycle; no vehicle more playful, no piece of exercise equipment so liberating, and no symbol of childhood that so powerfully and paradoxically signals the coming of adulthood.

Your bike takes you down the driveway; over the curb, way from the steady hand of your father, then beyond arm’s reach and, eventually, farther. As far as you want to go. The significance is not just the distance that grows between the hand and the bike, but the speed. Even wobbly, unsure, turning to look over your shoulder to see if you truly are riding on your own — even then you are, for the first time in your life, faster than Mom and Dad.

The bike, the first vehicle we master, teaches us the costs and consequences of propulsion: Fall and you scrape not jus your knee but your shiny new machine as well. Forget to inflate your tire and you get a flat. Leave the bike behind the car and it gets mangled.

Some people go so far as to equate the lessons of balance that we must master with, well, lessons of balance. “I finally determined that all failures were from a wobbling will rather than a wobbling wheel,” wrote Frances Willard, a turn-of-the-century women’s rights activist who learned to ride a bicycle at 50. It could be that when we learn just how much speed will take us around a corner safely, we’re learning something about how to manage a career 30 years hence. If so, it is not at all apparent in the moment, and barely there in hindsight, which makes it the best sort of cosmic lesson.

A bike is the one Christmas present every child receives, the birthday gift your parents shouldn’t have been able to afford, the unfulfilled dream in the shop window that comes true one time for all of us. A child on a bike is a wish fulfilled and a promise still in the making.

“As a kid, I had a dream — I wanted to own my own bicycle,” said John Lennon, in Bicycling magazine. “When I got the bicycle, I must have been the happiest boy in Liverpool, maybe in the world. I lived for that bike. Most kids left their bikes in the backyard at night. Not me. I insisted on taking mine indoors and the first night I even kept it in my bed.”

It’s appropriate that our parents give us our first bike, for it’s a metaphor of what they must do to raise us: Provide us with the tools we need to leave them. And though the process of learning to ride seems to be almost guaranteed — more of us can ride bikes than swim — it’s impossible to overstate the difficulty of what we actually accomplish when we pedal off on our own.

“The machine appears uncomplicated but the theories governing its motion are nightmarish,” bicycle physicist Chester Kyle explained to Bicycling. “Some things can’t be easily defined by physics and mathematics. The interactions of the body, mind, muscles, terrain, gravity, air and bicycle are so complex that they defy exact mathematical solutions. The feel and handling of a bike borders on art. Like the violin, it’s been largely designed by touch, inspiration, and experimentation.”

A bicycle is a remarkable feat of engineering — it can carry ten times its own weight and uses energy more efficiently than a soaring eagle. Yet a seven-year-old can master its mechanics. Indeed, it’s the first machine many of us ever take apart and successfully (or not) reassemble. There’s something about its lines, some feeling inherent in its circles and curves, that appeals to us. Our longing for a shiny new bicycle lives somewhere beyond practical. Toys hit children in waves of popularity and resurgence: Count on the yo-yo to become hot again every decade or so, and hold onto your scooters for the next revival.

The bicycle never goes out of fashion. A first bicycle lives in us like a first kiss. A best friend. our favorite dog.

Mike Burrows, a designer who’s created some of the most stunning modern bikes, admits: “Some things need to be drawn before they can be designed and understood. Others need to be made first, and the bicycle is the latter,” he says. “It is inconceivable that the principles involved in riding a bicycle could ever be theorized first. It’s far more likely that the principles of balance related to the bicycle were discovered by someone playing around with things that had wheels. Put simply, a cyclist proceeds in a series of falls that are compensated for by steering the bicycle back under the center of gravity. This complex principle cannot be analyzed by computers but is done automatically by us, clever apes. And it is a skill that once learned is never forgotten.”

We pine for our lost bicycles. Those that are stolen teach us a whole other lesson about the adult world we’re riding so carelessly into. A bike taken is a bike forever elevated in memory. The story is different for the bikes we abandon, which is what happens most often. We leave our bikes as inevitably as we leave our childhood.

I’ve often wondered if children can sense all that’s in a bike, if that sort of ancestral knowledge might explain our attraction to these strange vehicles. Is this a modern version of an ordained pairing like caveman and canine? Perhaps the power lies not in the bicycle itself but in the adults who pass on the artifact; maybe we imbue the bicycle with something of what we know. In this way, riding a bicycle is like crossing a bridge from childhood to adulthood. A kid can pedal across the bridge as often and as far as he or she wants and always return back to being a kid — not so once you drive across that bridge with a car.

Sometimes as adults, we end up figuring out how to sneak back across that bridge — but only if we find our way back to a bike. If we’re very lucky and rediscover cycling when we’re older, the bike gets a chance to perform its miracle again, but in reverse. Just as the bike lets the child glimpse adulthood while remaining resolutely a child, so an adult on a bike gets to sightsee through youth. The appeal is not, as it originally was, the magnificence of the distance we can achieve, but the intimacy of the trip. As adults, we might never ride our bikes farther than we drive to work (and certainly few of us ride our bikes farther than we drive in a single day) but a simple 5-mile spin through the neighborhood can take us much father than we actually traveled. It’s not so much the unknown world that beckons as the freshness of the familiar world you’ve come to inhabit. You become acquainted with cracks in the road, with curbs, with dogs that confront you. You run your wheels across skittering leaves, drop your head and milk the speed of a fine downhill. Ride a bike through your old neighborhood and you can almost hear your mother calling you in for dinner.

Ride for an hour and you burn enough to enjoy the extra slices of cheese with the wine. Ride through a Saturday afternoon and you no longer have to think twice about that night’s triple-fudge sundae — you can indulge as guiltlessly and guilelessly as a child. Ride most of the Sundays through a year and you regain the metabolism of a child, the unthinking ability to incinerate whatever’s put in front of you.

A bicycle moves at the ideal pace to see the world: Fast enough to outrace boredom, slow enough to absorb detail. On a bike, you become part of your environment rather than hurtling through it in a car or plane. You can dawdle or blur your eyes with speed and either way be confident that you’re moving at a human pace.

The bicycle is an equalizer; it opens it magic to any of us. Its frame supports people too heavy to even walk or jog; its smooth circular motion soothes damaged knees and welcomes those who can’t participate in impact or torsional sports; blind people hop onto the backs of tandem bikes for the thrill of the ride.

Like all aerobic sports, cycling releases endorphins. But there’s some evidence that the rhythm of pedaling itself helps the brain mimic the calming and restorative state of deep meditation.

Stephen Crane, Ernest Hemingway, and Pablo Picasso all loved bikes through the lives (and put cycling into their art). Albert Einstein said he thought of the theory of relativity while riding his bike. Simone de Beauvoir told of Jean-Paul Sartre that he “much preferred riding a bicycle to walking. He would amuse himself by sprinting on hills. On level stretches, he pedaled with such indifference that on two or three occasions he landed in ditches.”

We never forget how to ride a bike, so the saying goes, and it’s so true that it seems an oddity of humanity: Why does this particular machine hold such a spot in our souls? Stop chasing fly balls for a year or two and you’re useless in the outfield at the softball game. Just try to swing among tree limbs like you did when you were seven. But abandon your bike, banish the idea of cycling from your life for 20 years, 30 years, then pick up a bike, throw a leg over it, hop on, and pedal off. It just feels right.

“To ride a bike is very like a love affair,” wrote H.G. Wells in his cycling novel, The Wheels of Chance. “Chiefly, it is a matter of faith. Believe you can do it, and the thing is done; doubt, and for the life of you, you cannot.”

Then, of course when it’s time for us to pass along the love of cycling, we see the lesson from the other side of the bridge. To run alongside a wobbly but speedy child on a bike is to confront the conundrum of parenting: How much support and how much freedom? In which balance lies success?

Faith? Love? Freedom? Is this too much to ascribe to a simple machine on which we are both engine and fuel (as well as passenger and pilot)? Can there be so much that’s so elemental in something designed most exquisitely to zoom down hills? The answer is right there, in your bike right now. Put a leg over one, and you can instantly see across that bridge from adult to child. There are about 1.1 billion bicycles making that trip right now. A billion people are pedaling with freedom and joy and innocence on the grandest, noblest toy in all the world.