When I was a child, we like to play Hacky sacks filled with sand or corn grains. It is somewhat not hard like sand and corn.
Today, I find it is because of the a special characteristics of a Fouth-phase in nature: granular matters, added to solid, gas and liquid. When you drop a single glass bead on a floor, it bounces up again and again. However, when you put more of beads in a sack and drop it to the floor, the sack will not bounce back. The energy is dissipated by the collision between beads. It is a kind of collective behaviour.
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