A bicycle, often called a bike or cycle, is a human-powered, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A bicycle rider is called a cyclist, or bicyclist.

Bicycles were introduced in the 19th century and now number about one billion worldwide.They are the principal means of transportation in many regions. They also provide a popular form of recreation, and have been adapted for such uses as children?s toys, adult fitness, military and police applications, courier services, and competitive sports.

The basic shape and configuration of a typical bicycle has changed little since the first chain-driven model was developed around 1885.Many details have been improved, especially since the advent of modern materials and computer-aided design. These have allowed for a proliferation of specialized designs for particular types of cycling.

The bicycle has had a considerable effect on human society, in both the cultural and industrial realms. In its early years, bicycle construction drew on pre-existing technologies; more recently, bicycle technology has, in turn, contributed both to old and new areas.

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