The Museum of Unworkable Devices is a fun place to visit. Within it ther are several places like the Gallery of Ingenious, but Impractical Devices This site is fun and irritating. I know just enough to believe that some of these devices should work and just enough to know why they don't.
In the 7th grade I did a science project on perpetual motion where I hooked a generator to a motor. The generator was started by a crank and was supposed to power the motor which would in turn power the generator and so forth. I was very steamed that I could not get this to work.