A bicycle is a two-wheeled machine that you ride by sitting on it and pushing with your feet on pedals attached to the front wheel. It also has a handlebar for steering and a saddlelike seat. You can drive a bicycle by pedaling, or you can use an electric motor to assist in the process. Bicycles come in many different styles, and you can customize them with a variety of accessories. You can find bicycles with coaster brakes or hand brakes, and you can choose a single-speed or multiple-speed drivetrain.

Bicycles have made an important impact on human society, both as a form of transportation and as a recreational activity. Around the turn of the 20th century, they helped to reduce crowding in urban tenements by allowing people to commute from more spacious dwellings in the suburbs. They also reduced the need for horses to haul heavy freight, and they were three to four times more efficient than walking in transporting people and goods.

Modern bikes have frames of steel, aluminum, titanium, or carbon fiber. These are lighter and stronger than older metals, and they can be built to be as light or as rugged as you want. Many professional racing bikes have carbon frames, which are the most lightweight and stiff. They can cost up to $4,000.

Most bicycles have coaster or hand brakes. Coaster brakes are activated by a rider pedaling backward, while hand brakes are controlled with levers on the handlebars. When the rider squeezes the levers, pads, located on each side of the frame, apply pressure to the wheels and stop the bicycle. Some bicycles have gears, which are sets of chains that link the pedals to the rear wheel. Shifting these lets you keep a consistent pedaling speed while traveling on different surfaces, with higher gears making it harder to pedal but faster than lower ones.

In the United States, a bicycle with fixed gears is often called a fixie because there is no shifting. These are usually used by children and casual cyclists, and are easier to operate than more complicated models with derailleur gears.

Bicycles can be customized with a variety of accessories, including lights, reflectors, mirrors, racks, and trailers for carrying cargo. More technical accessories include cyclocomputers that record speed, distance, and other data, and clipless pedals that allow riders to keep their feet in the correct position on the pedals. There are also specialized clothing and footwear designed for bike riding, including helmets.

There are a number of specialized bicycles for people with physical disabilities or limited mobility, including wheelchair users. They have rear racks and other accommodations for carrying cargo, and some even have electric motors to help propel the bicycle along. Other options include recumbent bicycles, which are similar to upright bicycles but have a seat that allows the user to sit in a reclined position. They are often preferred by people with nerve, skeletal, or muscle problems that prevent them from riding an upright bicycle.