Milk is an emulsion.
Many people enjoy peanut butter which is a mixture of peanut oil and peanut water soluble compounds. Homemade peanut butter will separate into two layers that need to be peroiodically remixed. Store bought has special emulsifiers to prevent this annoying occurance. If milk was not an emulsion, the mother would have to suppley two different kinds of milk. Argonne National Laboratory, Division of Educational Programs, Harold Myron, Ph.D., Division Director.