The bicycle is a human-powered, two-wheeled vehicle that can be used for recreation or transportation. A bicycle has handlebars for steering, a saddle for sitting on, and pedals to provide power for movement. Some bicycles have gears, which allow the rider to change the amount of effort needed to pedal and to move at different speeds on different surfaces. Thousands of people die every year in bicycle accidents, but careful riding can help prevent them. Many cities and states have laws requiring cyclists to wear helmets and obey traffic signals and signs.

The earliest known bicycle was created in 1817 by Karl von Drais, who developed a machine that allowed the rider to propel himself by pushing his feet against the ground. The modern bicycle is based on Drais’s design, and it uses pedals to convert the rider’s motion into mechanical energy. The pedals are connected to cranks that drive chains and gears, which then drive the wheels.

Most modern bicycles have upright seating, and the frames are built using a diamond truss that includes a head tube (which contains the headset), a top tube, a seat tube, and paired chain stays and down tubes. The frame is usually made from steel, but it can also be made of other materials such as aluminum or titanium. Some bikes use a suspension seat post or suspension seat spring to improve comfort by absorbing shock from the road or other rough terrain.

Many people use bicycles to get around, particularly in urban areas where there are few public transit options. The low cost and convenience of bicycles make them a popular mode of transportation. Many people also use them for exercise, to keep fit or lose weight, and to compete in sporting events such as races.

Some bicycles have brakes to stop the wheels from turning, and these may be powered by the rider’s hand or by a lever on the handlebars. Some have gears to adjust the pedaling speed to match the terrain or the load, and these are called derailleurs.

Bicycles can be fitted with accessories such as baskets to carry goods, and racks to hold luggage or a backpack. Parents may add rear-mounted child seats or auxiliary saddles to the crossbar to transport children. Many cyclists use lights to increase visibility, and some even have specialized reflective clothing to increase their safety. A bicycle can be equipped with a hitch to tow a trailer carrying cargo or a person. Various types of bicycles are available, with the most common being the road bicycle that is designed for use on paved roads. Other bicycles are designed for off-road use, competition, or for touring and leisure. People who enjoy these activities may join clubs to meet others with the same interest. This can lead to friendships and even romance. Some professional cyclists earn a living by competing in races, and some have risen to international fame. Many other people simply enjoy the pleasure of riding a bike.