A bicycle, or bike, is a pedal-powered two-wheeled steerable vehicle. Bikes have been used for both transportation and recreation since the 19th century, and more than a billion of them are now in use around the world—more than twice as many as automobiles. The sport of cycling, or biking, has become popular worldwide as a means of exercise and transportation, and it is also a very efficient form of transport for long distances.

The word “bicycle” derives from the Latin words for two (bi-) and circle (kulos), so the phrase literally means “two-wheeled horse.” In addition to its primary uses, a bicycle is widely used as a recreational tool and for racing. The sport of competitive biking, or road racing, has produced some of the world’s most famous athletes, including Eddy Merckx, who won the Tour de France five times.

A modern bicycle is a complex machine, but the basic design has changed little since the first chain-driven model was developed in 1885. It consists of two wheels of the same size attached to a frame that is supported by a rotatable fork. The rider sits on a saddle and steers by leaning and turning the handlebars, which are connected to the fork via a tangent linkage. The feet power the bicycle by pedaling, which drives a chain that connects the pedals to a sprocket on the rear wheel.

Modern bicycles can be fitted with gears, which allow the rider to climb hills easily or race at nearly the same speed as a car. A typical bicycle has a front chain wheel with 44 teeth and a rear gear with 11 teeth. At a pedaling rate of 60 rpm, the chain moves 326 inches with each stroke, and the top speed for a bicycle with 26-inch wheels is about 37 mph.

Cycling provides a great source of exercise for people of all ages and abilities. It helps to strengthen muscles and bones, improves balance and coordination, burns calories and reduces stress. It also can help prevent osteoporosis, because it is not a weight-bearing activity, and it may even relieve symptoms of depression and anxiety.

There are some risks to cycling, especially for those who ride aggressively or who have a history of injury or illness. These include injuries to the wrist, which are caused by the pressure of the handgrips on the handlebars; muscle cramps in the legs, arms and back, which can occur from riding for prolonged periods of time or as a result of improper training techniques; sores on the buttocks, which are often caused by pressure on the saddle; and hip pain, which can occur because of repetitive motion of the soft tissues against the underlying bone.

Other risks are more serious, such as collisions with motor vehicles or falls while riding on unpaved trails. For this reason, it is important for all cyclists to wear a helmet when riding, obey traffic laws and be vigilant when sharing the road with other vehicles.