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 always in motion when the bike is moving, allowing the rider to control speed and braking by pedaling backward. Fixed-gear bikes are popular for their simplicity and lightweight design. These bicycles are commonly used in urban environments and by track cyclists due to their efficiency and direct power transfer. Riders often appreciate the minimal maintenance required, as there are fewer components compared to multi-gear bikes. However, they can be challenging to ride on steep hills.