The bicycle is a small human-powered vehicle that uses momentum, force and friction to get riders from point A to point B. It is the most efficient means yet devised for converting human energy into motion. The seemingly simple two-wheeled machine harnesses physics and mechanics to provide effortless mobility for millions of people around the world for transportation, exercise, recreation and fun.

The bike consists of a frame, two wheels, a metal chain connected to cogs on the pedals and rear wheel and a seat with handlebars attached to the frame. A rider sits on the seat and powers it by pushing down on the pedals with his or her feet. Other bicycle parts include a crank and chain, handlebars, a headset and brakes.

There are many theories about the origin of the bicycle, and no single individual qualifies as its inventor. Among the many presumed ancestors of the modern bicycle are the boneshaker, velocipede and penny-farthing.

The modern bicycle was revolutionized by the invention of the safety bicycle in the late 1880s. It used smaller front wheels, lowered the seat height and improved the balance of the rider’s weight. It also introduced pneumatic tires and a drive system that allowed the rider to both pedal and steer via the handlebars. This design, along with the development of gearing, made the bicycle a practical mode of transportation for many people.

Choosing the right frame size is one of the most important decisions for a beginning or casual cyclist. A bicycle’s frame is its backbone, so it must be strong enough for a rider of any age or fitness level to safely use. Most bicycle frames are crafted from steel or aluminum. Aluminum alloys are lighter and stronger than steel, but the materials must be carefully selected to achieve the required strength, stiffness and ride quality.

Other than a bicycle’s frame, the most common components are a chain, grips and saddle. The latter is where the rider makes contact with the bike and can be adjusted for comfort by changing its shape and position. The rider can also change the gearing by adding or removing chains from the crankset, changing the ratio of the drive system and turning the chain adjuster to control speed.

There are several other optional bicycle components that can be added for special purposes. A bicycle can be fitted with a hitch to haul a trailer for cargo or children, or with an auxiliary saddle that fits over the crossbar or between the rider’s arms. Some bikes can be fitted with pegs that allow a rider to perform tricks or practice certain skills, while others are built for racing and have specific peg locations that are matched to race-style wheels.

The road trip is an American cultural icon that grew out of the confluence of post-World War II trends. The automobile completed its transformation from a utilitarian tool to an iconic symbol of personal expression and freedom, while a generation of restless youth sought adventure, travel and identity. By the mid-1950s, the U.S. Federal-Aid Highway Act started construction of the interstate system and motels dotted the map across the nation, making the road trip more accessible to average Americans.