Fixed-gear bicycle
Most bicycles have a freewheel so the pedals will stay stationary while the bicycle is moving and so that the rider can ride without pedaling going forward or down a hill. Fixed-gear bicycles have even simpler mechanisms with pedals fixed to the back wheel. That means that the rider cannot stop pedaling. When the rear wheel turns, the pedals turn in the same direction. This allows a cyclist to use a brake by putting a pressure in opposite directions to pedals, so that the mechanism will not rotate.