Milk Plastic Tags #DIY. Plastic from milk? Sounds crazy, but it’s true! Before modern plastic was invented, this type of plastic was used to make things like buttons, beads, and pens. What’s going on? Were you ever told, “Drink your milk, it’s full of protein”? Well, that protein, called casein, is what you turned into plastic. Casein is a polymer, and up close, casein molecules look like squiggly balls.The vinegar makes the squiggles unfold, stretch out, and grab onto one another.