Rice is the most processor-intensive default material in the game. This is because it is a combination of three materials - Rice, Elastic, and Viscous, all of which are also processor-intensive on their own. Rice may be an attempt at simulating a Maxwell material.
Rice causes String and Elastic particles to not bond to each other, making every particle an individual. Rice appears to stick to itself due to the Viscous in the mixture, which stops the particles from sliding around freely. The Elastic makes the mixture very bouncy and springy.
Elemental Rice (pure Rice, made by pressing esc-K-esc) is a slightly bouncy, springy, squishy fluid like Water. It does not flow smoothly, instead it bounces and splatters everywhere. Rice has unique programming compared to other materials, it has special powers that allow it to create very unconvential reactions. Rice mixed with Powder makes it flow a bit like a non-Newtonian fluid, where it will flow smoothly and gently, but becomes more solid when it is hit. Rice is relatively untested and is sure to have more secrets.