We’ve mentioned making plastic with milk before; Coffeebot has a great Instructable on the subject.
The plastic is moldable, and has a consistency of soggy cheese (I certainly hope I never encounter cheese that’s anything like this!). When all is said and done, it should take you about 10-15 minutes to make the plastic (less if you make a small amount), 10 minutes of cleanup, plus 2+ days to allow the plastic to dry. As always, your milage may vary.
Casein is the main protein in milk, and it’s commonly used in adhesives and paints. And knitting needles! (I don’t think my cat could resist those.) It was the main ingredient in an early plastic called galalith – it was mixed with formaldehyde, nasty stuff.