Why do eggs become hard on boiling?

Question: Why do eggs become hard on boiling?
- Keywords:
  boiling,
  denature,
  eggs,
  hard boiled,
  protein
Asked by saraw1998 to Arttu, Ceri, James_M, Monica, Philip on 13 Jun 2011.
- Arttu Rajantie answered on 13 Jun 2011:
  
  The egg is mostly made of protein, which consists of long molecule chains. In a raw egg these chains are curled up into tiny balls. Therefore they can move past each other easily and behave like a liquid. When you boil the egg, the proteins uncurl and get all tangled together, so they all form one solid lump.

Question: Why do eggs become hard on boiling?