Oil skimmers are pieces of equipment that remove oil floating on the surface of a fluid. In general, oil skimmers work because they are made of materials to which oil is more likely to stick than the fluid it is floating on. At the same time, the fluid has very little attraction to oil skimmers. Oil skimmers are usually all that is necessary to remove oil from a liquid. In some cases, however, oil skimmers may be used to pre-treat a fluid. In this case, the oil skimmers remove as much of the oil as possible before more expensive and time-consuming measures are employed. Pre-treating the fluid with oil skimmers reduces the overall cost of cleaning the liquid.