Eggs contain protein and proteins are a polymer in amino acids. When an egg is boiled the protein present inside the egg is denatured(the process of loss of biological activity of protein like their ability of –H bonding when subjected to a physical change like a change in temperature, pH) and coagulated(the process of liquid changing to a solid or semi-solid state). Due to this, the water present in the egg is absorbed by the coagulated protein by –H bonding.