how to make gears
When you turn the 3-inch gear clockwise, the 1 ½-inch gear turns counter-clockwise. When you turn the 3-inch gear once, the 1 ½-inch gear goes around twice. When you turn the 1 ½-inch gear once, the 3-inch gear makes half of a rotation.
Gears transmit torques (twisting forces) in predictable ways—this is why they are so useful in machines that require exact movements, like clocks. The 3-inch circle made the 1 ½-inch gear spin around twice because the 3-inch gear has twice the circumference.