Heated milk, vinegar and a strainer= casein from milk which is able to be molded and hardens in a few days. Ta da!= 'plastic milk!' 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. Easy and close enough!
Another big THANK YOU to my senior school helpers:)