DriverFixed to the 36 behind itBack layerTop: a simple train. Every gear sits on its own shaft, so only the first and last tooth counts matter (24/12 = 2). The 20-tooth idler in the middle doesn't change the ratio, it only reverses the direction. Bottom: a compound train. The red 12-tooth gear is fixed to the first 36 and sits in front of it, so the two stage ratios multiply: 3 × 3 = 9 with only four gears. Watch the dot on each gear to see how fast it turns.