A bicycle (or bike) is a human powered two-wheeled vehicle that’s propelled by the rider’s feet and steered by handlebars. It is used by millions of people around the world for recreation, exercise, sport (called cycling), and as a means of transportation. Bicycles are built with a frame, a seat, and two pedals connected to a chain that runs from the seat to the back wheel. When the rider pushes down on the pedals, the chain turns the cogs on the rear wheel and the bicycle moves forward. The rider can control the speed and direction of travel by turning the handlebars or leaning in a particular direction.

Early bicycles were called velocipedes or draisines and had wood, iron or wire-spoked wheels. Their hard, jarring ride gave them the nickname “boneshakers.” Development of the pneumatic tire and the chain drive, in the late 1800s, improved bicycles’ quality and made them more practical for transportation.

A modern bicycle’s frame is usually made of steel or aluminum, and it may be formed into a variety of shapes for different purposes. Suspension seat posts and seats can make a more comfortable ride for longer distances, while mudguards and racks help keep the wheels clean and provide a place to carry luggage or groceries.

There are several basic styles of bicycles: upright, or “recumbent,” and recumbent tandems. Both types have handlebars for steering, but upright bikes also have saddles that can be adjusted to the rider’s height. Upright bicycles have a long-wheelbase design that’s best for road riding, and short-wheelbase designs that are more suited to off-road use.

The frame of a bicycle contains a set of ball bearings in which the yellow ones ride, and a light blue tube holds cones that screw on to the wheel hubs. These cones help adjust how tightly the wheels, or hubs, are held by the bearings. A little bit of grease in the hubs can make them run more smoothly.

A bicycle is the most efficient human-powered machine for transportation, in terms of how much energy it requires to travel a given distance, although gearing can improve this efficiency by up to 10%. It is the most efficient means of transport for carrying cargo, as well. However, bicycles are not without their drawbacks. They are noisy, require regular maintenance and repair, and can be dangerous for riders who do not follow traffic laws or possess a limited knowledge of bicycle safety. Nevertheless, they remain the most popular form of transportation in the world. Drivers should yield to bicycles, reduce their speed for road conditions and always look for them before turning, especially in parking lots. They should be aware that cyclists often leave little to no time to brake before a stop sign or when changing lanes.