A few years ago I had an idea for a perpetual motion device that I was convinced would actually work. Now, I know that the first rule of thermodynamics doesn't allow "Free" energy and I know every perpetual motion device in history was a hoax but I couldn't find an explanation WHY my device wouldn't work. It's a very simple device - Imagine a looped rope with empty barrels attached to it, One side of the loop is submerged vertically in water and the other side is outside of the water. The barrels that are submerged would float up, while the barrels in the air would fall down, and then enter the water from the bottom and float back up.

It sounded so perfectly reasonable but I knew the rules of physics wouldn't allow it somehow. And you know what it is? what makes this concept unrealistic? It's the surface tension of the water! Trying to push a barrels back into the water would require more energy than the machine would put out! Dammit physics! you win again!