A bicycle is a small human powered land vehicle that has two wheels, a seat, and handlebars for steering. Millions of people worldwide ride bikes for fun, exercise, or transportation. A bicycle’s basic structure has changed little since its development in the 19th century, though modern materials and computer aided design have allowed for a wide range of specialized designs.

The bicycle is the most efficient human-powered means of transport, in terms of the amount of energy a person expends per unit of distance traveled. A typical bicycle transfers up to 99% of the energy put into the pedals into forward motion, with the remaining fraction used for balance and control.

Bicycles are built from a variety of materials, but most modern bicycles feature lightweight aluminum alloy frames and components. The use of these materials makes bicycles much more affordable than automobiles and other forms of motorized transportation, while also allowing designers to make lighter weight, stiffer, and more compliant vehicles.

The word “bicycle” is derived from the Latin words for two and wheel, and it has come to be the world’s most common means of transportation. Before the invention of the bicycle, humans moved around on foot or in animal-drawn carriages. In the 1860s, inventors began to add pedals to these animals, giving them greater speed and maneuverability. These early machines were called dandy horses, boneshakers, and velocipedes, but the term bicycle became popular by 1882, when the English writer Henry Lawson applied it to the machine he invented.

Other inventions soon followed, including better tires (which can be pneumatic or solid), better hand brakes, and a freewheel device that allows the back wheel to spin even when the rider is not pushing on the pedals. The freewheel device also enabled more complex gearing, allowing riders to travel over hills with less effort.

A bicycle may be equipped with a number of accessories for safety and comfort, such as headlights, reflectors, and fenders. Technical accessories, such as cyclocomputers, are also available to help riders monitor and optimize their performance. Some bicycles are fitted with hitches to carry cargo or a child seat.

The bicycle has become so common that it is now a very diverse form of transportation. People ride bicycles for recreation, in competitions such as BMX racing and criterium, and for fitness and transport. It has also become an important part of the infrastructure in some developing countries. Its widespread use has also resulted in the growth of related industries such as bicycle parts manufacturing and cycling tourism. In addition, the bicycle has helped develop advanced metalworking techniques that were later adapted for use in other technologies such as automobiles and aircraft.