Mechanical clocks tell time by using gears. They have two important parts. The first part is a mainspring and the second part is a pendulum. Mechanical clocks are wound with a key, and this tightens the mainspring. As the mainspring unwinds, its energy turns gears which cause the hands to move. The pendulum keeps time and ensures that the gears move at the right pace: second by second. Instead of a mainspring, some mechanical clocks have weights that pull the gears at the right step. Mechanical clocks do not need electricity to operate. They can run off of the energy generated by their springs and weights.