A ratchet mechanism is based on a wheel that has teeth cut out of it and a pawl that follows as the wheel turns. Studying the diagram you will see that as the ratchet wheel turns and the pawl falls into the 'dip' between the teeth. The ratchet wheel can only turn in one direction - in this case anticlockwise.

The water well seen opposite has a ratchet mechanism that allows the person to rotate the handle in an anticlockwise direction. The bucket of water is heavy and so the person can rest by taking his/her hands away from the handle. This is because the pawl has fallen into the 'dip' between the teeth and so the bucket cannot fall back into the well.

Ratchet mechanisms are very useful devices for example, they are used in mechanical clocks. They are also very useful when using a system, such as the one seen opposite, to lift heavy weights.