Why does an egg get hard when you boil it?

Answer 1

Eggs are rich in protein, especially the egg whites. It's this protein that causes eggs to become hard when boiled. Here's how it works:
Hint
If you don't use a hard-boiled egg (which you must keep in the refrigerator) within seven days, toss it!

Protein is a chain of amino acids. These amino-acid strings fold back on themselves (the way the strings fold determines the protein's chemical and biological properties). The proteins are held in place by weak bonds (non-covalent) between different parts of the amino-acid string. When you break those strings, by various methods, you are denaturing the protein.

You can denature a protein in several different ways. Here are two ways to do it with eggs:

Heat - 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 temperature increases, the proteins gain enough energy to form new, stronger bonds (covalent) with other protein molecules.
When you boil an egg, the heat first breaks (unfolds) the proteins, and then allows the proteins to link to other proteins. 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. (Heat affects all of the proteins in an egg, so it's the best way to cook an egg.)

Answer 2

you have boiled it too long. Bring to boil for 10 minutes and take off burner. Let sit for 10 minutes. Run cold water over eggs, then peel.

Answer 3

I usually do 8-10 minutes. I should add that you have to put the eggs in the water and THEN bring it to a boil. This might seem like common sense but I know people who have dropped eggs into boiling water and wondered why their eggs were exploding. So. Yeah.

Answer 4

It gets hard because it was a Hot Chick


http://www.starcarlton.com/Flash/ILoveEggRobbery.htm

Answer 5

Christian has the right answer

Answer 6

because you have cooked it.