A complex machine is a combination of two or more simple machines that work together, such as when a lever and a wheel and axle are combined to form a cart. There are many different possible iterations of complex machines, from staplers to wheelbarrows and skateboards.
Simple machines are a group of six items (wheel and axle, pulley, screw, lever, wedge, and inclined plane) that can be combined to form a complex machine. A bicycle is an example of a complex machine because it involves the use of two wheels and axles. A dolly, or handcart, is another example, with a lever and wheel and axle being combined to help lift and move heavy loads. The term "complex machine" may be used interchangeably with "compound machine," which also refers to a group of multiple simple machines working together to make a more complicated mechanism.