different materials have different densities. The red ball is stiff, probably a metal ball. The blue one is fluid and changes shape due to gravity, like a water balloon. Its important to have these because you can´t explain every single item in an animation, so you have to make it obvious. if you want to show that this ball is a water balloon, then let it flow and change shape. If you want to show if something is metal, keep it still when in motion.