You can also view previously answered questions in the HSW Question Archive . Answer Eggs are rich in protein , especially the egg whites. It's this protein that causes eggs to become hard when boiled.
Protein is a chain of amino acids .
When you break those strings, by various methods, you are denaturing the protein . You can denature a protein in several different ways. When you heat an egg, the proteins gain energy and literally shake apart the bonds between the parts of the amino-acid strings, causing the proteins to unfold. As the proteins form these new, strong bonds, the water that surrounded each protein molecule when the egg was liquid is forced out. That's why the egg turns hard.