If you leave the "cow juice" set in a pan for a long period of time, the cream rises to the top because the milk is heavier. The typical separator substitutes a large amount of centrifugal force for gravity, flinging the milk to the outside, where it is collected and the cream stays in the middle and is collected. The series of discs, with very specific heights of the ridges or dams determines the actual separation point. Different size dams will give different percentages of milk and cream. Same technique is used to separate water (heavy) from steam turbine lube oil (light)
