A combination of rigid or resistant bodies having definite motions and capable of performing useful work. The term mechanism is closely related but applies only to the physical arrangement that provides for the definite motions of the parts of a machine. For example, a wristwatch is a mechanism, but it does no useful work and thus is not a machine. Machines vary widely in appearance, function, and complexity from the simple hand-operated paper punch to the ocean liner, which is itself composed of many simple and complex machines. No matter how complicated in appearance, every machine may be broken down into smaller and smaller assemblies, until an analysis of the operation becomes dependent upon an understanding of a few basic concepts, most of which come from elementary physics.