Here is a Gear Train. Hold the handle and rotate it slowly. When you complete a rotation of the handle, observe the number of rotations taken by the other gear.

What did you observe? 
For one complete rotation of the gear, the second one completes nearly 3 rotations.

The science behind it :
The gear wheels exhibit the relationship of speed and torque. Such gear trains are used in the machineries to increase the speed of torque.