Inside a standard mechan- ical clock, the two cogwheels on the left are attached to two axles.The two cogwheels on the right are fixed on the same axle, and move together. Notice that the two cogwheels at the front (left and right) are in contact, and that the same is true for the two at the back. The clock mech- anism rotates the inner axle on the left. The back cogwheel on the left is attached to this axle, and so turns with it and also makes the back cogwheel on the right turn (in the opposite direction).