Why do eggs become hard on boiling?

Question: Why do eggs become hard on boiling?
- Keywords:
  egg
Asked by meg123 to Amy, Karen, Sarah, Vijay, Will on 18 Jun 2012.
- Sarah Martin answered on 18 Jun 2012:
  
  Hi again Meg! It’s because the proteins in the egg unfold when they’re heated up, and then they all stick together and instead of just swimming around in the liquid egg they form a web of stuck together protein that becomes hard!

meg123 commented on 18 Jun 2012:

Sarah! You have answered loads of my questions! You are great and thata was so interesting! Why is the yolk always in the middle though?

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Question: Why do eggs become hard on boiling?