Oobleck is a non-Newtonian fluid made from 2 parts corn starch to 1 part water. It is also based off a substance in the book
Well thats great but what is a non-Newtonian fluid you ask? It is a fluid that is not described by a single constant value of viscosity. With Oobleck it acts as a liquid when no force is applied, but if you try and hit it, it turns to a solid.