Plastic? In milk? Well, sort of. You made a substance called CASEIN. It’s from the latin word meaning “cheese.” Casein occurs when the protein in the milk meets the acid in the vinegar. The casein in milk does not mix with the acid and so it forms blobs. True plastics, called polymers, are a little different. If you want to make a true plastic and learn more about polymers, try the Homemade Slime experiment. Have fun!