Bicycles are used by millions of people to get to work or school, exercise, compete in races, and for fun. The bicycle is the most efficient means of human-powered transportation, converting up to 99% of the energy put into the pedals into motion. The bike is also a popular mode of recreation, entertainment, and transport for children, adults, and even whole families.

A bicycle has two wheels and a seat that can be adjusted to accommodate riders of any height. The frame and fork are typically made of a metal or aluminum, and most higher-end bikes are constructed of carbon fiber or other high-performance materials. There are many styles of bicycles, including racing, touring, and recreational models. Some have gears to allow the rider to vary their effort and speed. Others have attachments to carry cargo, and some are equipped with child seats or hitches to tow a trailer.

The International Organization for Standardization maintains a special technical committee (TC149) that is focused on cycles and their components. The committee defines terms, tests for safety and performance, and sets requirements for interchangeability of parts between different manufacturers. Bicycles may be built from any number of materials, depending on the desired cost and weight, and components such as tires are available in a wide range of colors, sizes, and shapes.

Riding a bicycle can be dangerous, and head injuries are responsible for the majority of fatalities and long-term disabilities in cyclists. To reduce these risks, all cyclists should wear a helmet and obey traffic laws. It is especially important to stay alert for road hazards, such as potholes, cracks, loose gravel, puddles, railroad tracks, and parked cars that might suddenly open doors or pull out from intersections.

Whether riding for pleasure, to get to work or school, to race, or to compete, cyclists need a reliable, safe bicycle. Regular maintenance and repairs are needed to keep the bike in working order. Many cyclists perform some of this work themselves, but it is always a good idea to have a professional inspect a bike before attempting any major repairs.

A bicycle is a very simple machine, and its mechanics are fascinating to people interested in machines and science. The frame and fork are primarily made of steel, which can be finished with paint or a durable coating. Aluminum is also sometimes used in the frame and fork, and titanium is often found in the most expensive bikes. There is no clear winner in the race to make the lightest and strongest frame, as there are tradeoffs between stiffness, weight, and comfort. Many different types of bicycles are manufactured, including recumbent bikes with a chair-like seat that some riders find more comfortable than a traditional saddle. In addition, people can add accessories to their bicycles such as baskets, racks, and lights. Some cyclists attach a rear-mounted child seat or auxiliary saddle to the crossbar to transport children, while others add a trailer for carrying cargo or a dog.