Milk is not turned into cream. Cream is a fatty layer of milk that will begin to separate and rise to the top of fresh milk that is left to sit for about 30 minutes or longer.
Natural creaming, which usually begins within 30 minutes of milk being obtained from a cow, takes about 24 hours to complete. Because the creaming process takes so long, the milk is no longer fresh when it is finished. For this reason, many dairy farmers use a machine called a centrifugal separator to speed up the process. Milk purchased in a store has already undergone the creaming process, so it is not possible to cream store-bought milk.