The bouncy ball granules are mostly composed of polyvinyl alcohol (PVA), which is a water-soluble polymer with elastic properties. Polymers are long, tangled chains of carbon, hydrogen, and oxygen atoms. When the granules are moistened, their surfaces start to dissolve slowly, they swell up a little, and they become sticky. The long polymers get tangled up in each other. In this way, the granules stick together inside the mould.
The balls are bouncy because the PVA material is elastic, which means it springs back to its original shape after being compressed or stretched. This elastic property comes from the shapes of the polymer chains. The long, tangled chains have extra space around them (think of the space between the strands of long noodles in a pasta bowl). When the force from hitting the floor causes the ball to compress, the polymers squish more tightly into the empty spaces. But at a point, the bonds between the polymers cannot compress anymore, and they start to expand again, releasing the force they absorbed, pushing the ball off the floor and causing the ball to bounce up.
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