It's fortunate that oil and water mix like, well, oil and water. When an oil spill occurs, the oil forms a millimeter-thick slick that floats on the water. The oil eventually spreads out, thinning as it does, until it becomes a widespread sheen on the water. How fast a cleanup crew can reach a spill -- along with other factors, like waves, currents and weather -- determines what method a team uses to clean a spill.