What is a key characteristic of perpetual motion machines?

A key characteristic of perpetual motion machines is that they defy the laws of thermodynamics. These machines are theoretical constructs that would operate indefinitely without an energy source, thus violating the first and second laws of thermodynamics. The first law states that energy cannot be created or destroyed, only transformed from one form to another, while the second law introduces the concept of entropy, indicating that systems naturally progress toward disorder and that energy transformations are not perfectly efficient.

In the context of perpetual motion, such machines would need to create energy from nothing or operate without any energy loss, both of which are impossible according to our current understanding of physics. Thus, the central feature of perpetual motion machines is their contradiction to these fundamental scientific principles.

The other options involve characteristics that either imply a form of energy dependence or efficiency that simply cannot exist in such machines, reinforcing the idea that perpetual motion concepts remain theoretically intriguing but practically unattainable.