The secret, water-absorbing chemical in a diaper is a superabsorbent polymer called sodium polyacrylate. A polymer is simply a long chain of repeating molecules. If the prefix “poly” means many, then a polymer is a long chain of molecules made up of many smaller units, called monomers, which are joined together. Some polymers are made up of millions of monomers.
Superabsorbent polymers expand tremendously when they come in contact with water because water is drawn into and held by the molecules of the polymer. They act like giant sponges. Some can soak up as much as 800 times their weight in water. Just imagine how much water a giant diaper could hold!
The cotton-like fibers you removed from the diaper help to spread out both the polymer and the liquid so that the baby doesn’t have to sit on a mushy lump of water-filled gel. It’s easy to see that even a little bit of polymer powder will hold a huge quantity of water, but it does have its limits.