Perpetual motion machines can do work indefinitely with zero energy input. If one existed, it could turn a wheel or raise water, thereby generating energy without needing fuel of any kind—a useful device. And in Medieval times, the scientists and engineers of the day thought it was possible, if you were clever and ingenious enough, to design and build them. Then, the First and Second Laws of Thermodynamics came along and proved such devices were not possible.
This did not deter everyone. There are still con artists who try to fool people into believing they’ve come up with a foolproof design; they just need funds to build a working prototype.And then there are the “mad scientist” types who truly believe they’ve found a loophole in the Laws of Thermodynamics.
Unfortunately, these true believers were doomed to failure before they ever started on their quest. There are also some academics who enjoy the mental challenge coming up with ideas and drawings of machines that look like they should work, to the uneducated eye.
Here is a collection from the past of various schemes put forth as perpetual motion machines. For more on this subject, check out the Museum of Unworkable Devices.