An account of the mechanical principles behind the design of gear wheels. The simple lever is illustrated by images of a woman operating a water pump, the oars of a rowing boat, an ancient stone relief, crowbars and a shaduf (a bucket on a lever counterbalanced by a weight used for lifting water). Animated diagrams bring out the mechanical principles behind the everyday examples. Images of raising water using a simple crank windlass, are followed by a four-handled windlass from Agricola's De Re Metallica (1556) and another animated diagram. The commentary suggests that the toothed wheel may have resulted from adding increasing numbers of arms to a windlass; this speculation is accompanied by another animated diagram.

The windmill, with its wooden gears - 'the typical machine of the Middle Ages' - is shown. The industrial revolution is presented as the next stage in the development of gear wheels because of the increasing use of metal in machines and the necessity of draining deep mines. This, the commentary explains, introduced steam pumping engines, then their adaptation to driving machinery in factories, the site of more intricate geared machines. The account concludes with synchromesh gears and modern lubricants. The explanations of epicycloid and involute gear profiles once again use animated diagrams accompanied by whimsical xylophone scales to demonstrate the principles behind the most efficient gear wheel tooth shapes.