Describe a perfect machine.
Its energy source is abundant, universally abundant. Its energy source, in fact, is surplus -- something that will not decrease or fail, and of which we may have too much. It has ideal energy properties -- it converts nearly all its input energy into output work, with almost no energy lost to heat.
It is constructed of inexpensive, widely-available materials. It can be made from recycled materials, and can itself be recycled. It requires almost no additional materials to function, beyond a little lubricant and the occasional replacement of worn parts. It can be maintained by its owner, and requires very few specialized tools to repair. If properly maintained, it has an almost unlimited use-life.
It is inexpensive. Almost anyone on earth can buy one.
Its use is democratic. Children as young as five can use it, as can very old people. It requires no license to operate. In almost all cases, if it is used improperly, its operator will sustain trivial injuries, requiring at most a few stitches.
It does not contribute to urban sprawl, global warming, air pollution, crime, obesity, or lung cancer. It will probably never require involvement in Middle Eastern conflicts. If we used this machine more frequently, all of these problems would improve.
It makes you healthier the more you use it.
The perfect machine is useful beyond description. It expands the world for its owner, and produces almost no harmful side effects. Its owner gains freedom of movement, independence. It can be used for transportation, exercise, haulage, recreation, nature appreciation. It exposes its operator to life.