When a bucket of water is at rest the water on the sides will rise for the following reason taken from my textbook: Water tends to cling to the walls of the glass because the adhesive forces between the molecules of water and the glass molecules are greater than the cohesive forces between the water molecules. In effect, the water molecules cling to the surface of the glass rather than fall back into the bulk of the liquid. In Brian Greene's book the Fabric of the Cosmos he says that when a bucket of water spins the center will rise. He never said why. It has to be something about the cohesive forces begin to become stronger when centrifugal force is applied but I can't think why.