From Household Hacker, How to Turn Milk Into Stone:
All we need for this simple kitchen science experiment is milk, vinegar and a strainer. You can make all kinds of shapes and designs using this “plastic-like” material. In fact, this is the stuff they used before plastic was around!
Take 1 warm cup of milk and mix with 1 tablespoon of white vinegar. Basically, for every cup of milk you use, add 1 tablespoon of vinegar and you can make a much larger batch. Mix it together and strain out the curds that form. Once you dry it you can mold it into any shape you desire, but let it dry for a couple days first!
Related: Make non-toxic glue from milk (and other household items)