Mechanical clocks tell time using gears. They have two important parts: a mainspring and 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 pace. Mechanical clocks do not need electricity to operate. They can run off of the energy generated by their springs and weights.