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My son and daughter got given gifts where you place a plastic egg in water, and the (rubber?) toy inside expands (over three days) and breaks the shell of the egg so it 'hatches'.

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I also observed that three weeks later the toys shrank again, but grew once placed in water.

I found the chemical reaction fascinating.

My question is: What is the reaction in a child's 'add water and watch the egg hatch' toy?

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The egg hatch toys are composed of a superabsorbent polymer. These polymers are able to absorb up to 30 to 60 time their volume in water by forming hydrogen bonds with the water molecules.

Sodium polyacrylate ($\ce{[-CH2-CH(CO2Na){}-]}_n%edit$) is the most common superabsorbent polymer and according to its wikipedia page, is the one used in the egg hatch toys.

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