A bicycle is a two-wheeled VEHICLE powered by pedals, which is also known as a VELOCIPEDE. It is composed of a frame, two wheels connected by spokes, and pedals connected to the rear wheel by a chain. The front wheel is usually larger than the rear wheel, and the rider sits between the two wheels. The rider can also be accompanied by a second person on a TANDEM bicycle. The term 'bicycle' is derived from the Greek words 'bios' meaning 'LIFE' and 'kuklos' meaning 'CIRCLE, ' referring to the circular motion of the pedals. RADII are the lines connecting the center of a CIRCLE to its circumference, which is often used to describe the shape of the bicycle's wheels.