A fixed-gear bicycle, often called a fixie, is a type of bike that has a single gear and no freewheel mechanism. This means that the pedals are directly connected to the rear wheel, so when the bike is moving, the pedals are always in motion. Riders cannot coast; they must keep pedaling to maintain speed. Fixed-gear bikes are popular among urban cyclists for their simplicity and lightweight design. They require less maintenance than multi-gear bikes and are often used for commuting or racing. However, they can be challenging to ride, especially on hills, due to the lack of gear options.