If you have a U-shaped pipe like the attached image, with more fluid in the left column than the right, then the fluid will level out basically because of Pascal's Law. Force on the fluid is exerted on every part of it in equal direction etc.. So when the fluid levels itself it flows across to the right hand side to fill the empty area. The fluid inside the bottom part of the U-shape flows to the right with a certain velocity or kinetic energy. The kinetic energy of that flow comes from the potential energy of the fluid in the tall left column. My question is what happens when you make the bottom of the U-shape very long? Now you are moving a lot more fluid with that little bit of potential energy. How is that energy being 'created from nothing'? I understand flow is not that simple and there are eddies etc.. But it must be true that fluid is basically flowing across the whole pipe, otherwise you couldn't fill that empty space in the right column.