In mechanical engineering there is the concept of a simple machine. As classically defined, there are six simple machines: the lever, the ramp, the wedge, the wheel and axle, the pulley, and the screw. The idea is either to change the direction of a force, or to provide a multiplier. With a lever, for example a crowbar, the long end moves a large distance with a smaller amount of force, while the short end moves a short distance with a lot of force. Mechanical engineers often use other simple devices to change direction or multiply: mechanisms like gears and gear trains, wheels with belts and chains, cranks and cams. Then there are springs and weights to store energy, motors and engines to add energy, etc. One place where all of these devices combine together in a visible, visceral way is the traditional water-powered gristmill. These mills dotted the American countryside in the 1700s and 1800s. Wake County, North Carolina, once had 70 such mills, one of which (Yates Mill) first opened in 1750 and is still in operation as a museum today. These mills used rotating millstones for grinding grain into flour. A wagon would pull up to the mill, using wheels and quite likely a ramp, to unload bags of grain. A rope and pulley would unload the bags and hoist them into a hopper. The power source for the mill was falling water, translated into rotational energy by a vertical overshot waterwheel. The waterwheel's rotating shaft would turn a large cogwheel, which would engage a small cogwheel. This wooden gear would shift the rotational force from horizontal to vertical and increase the vertical shaft speed. A belt system might come off one of the shafts to power things like an Archimedes screw or conveyor belt. A crank system or cam system might help create back and forth motion to clean the grain or sift the four. Think about any mechanical system you see today created by engineers: a car's engine, a sewing machine, a clock, etc. They're the same basic elements applied over and over again in myriad different ways. Simple machines form the foundation of mechanical engineering.