Generally speaking, machines are devices that make work easier or quicker, or, in some cases, possible at all. For example, an electric mixer makes the "laborious" job of mixing cake batter a lot less strenuous. Automobiles, too, make getting across town easier and quicker than walking. But machines don't have to be as complex as kitchen appliances or cars. Many other simpler devices also qualify as machines. A knife, for example, is a type of machine. So are screwdrivers, pulleys, bottle openers, baseball bats, screws, and countless other items people use without ever thinking of them as machines.

Some machines make tasks easier by providing what's called a mechanical advantage, reducing the amount of force you need to apply to achieve a particular amount of work. A crowbar, for example, makes prying a lid off of a box easier because it increases the amount of force being applied and changes the direction of the force; when you push down on the crowbar, the crowbar pushes up on the box lid with a greater force than you apply. Other machines -- a large gear on a bicycle, for example -- actually require a tremendous amount of input force to operate. In return, these machines can make a process go faster than it would without the machine.

Complex machines combine several -- sometimes many -- simple machines to carry out a single objective. In general, as with simple machines, most complex machines are designed to make tasks easier. A typical can opener, for example, employs at least four simple machines to make opening a can of soup a breeze. However, one peculiar type of complex machine strives to do the opposite.

Commonly referred to as Rube Goldberg machines, these devices make the simplest tasks unbelievably complex. In contests around the country, inventors go out of their way to solve problems, like sharpening a pencil, watering a plant, or pouring a glass of milk, in the most complicated and creative ways possible. Contestants are usually required to build at least 20 steps into their machines, but the more steps the better. Such complexity, of course, requires a lot of testing, both of the individual simple machines and of their interactions with one another.