Why do Eggs turn hard when you boil them?

 Eggs are rich in proteins, the three main components are the egg shell, the egg white and the egg yolk. All proteins are made of building blocks called as amino acids arranged in chains, the specific folding a chain of amino acid undergoes gives a protein its properties. 

Now this structure can be modified by various applications of heat or chemicals and the process of changing the original structure of a protein is called as denaturing.  Denaturation in simple words, alters the bonds between the amino acids chains in the original or native structure of the protein, altering the bonds alters the properties of the protein. 

In case of an egg, the egg white which is a semi-fluid state in its native form, on application of heat through boiling clumps together and hardens, forming the white exterior of boiled eggs, the egg yolk which is also in a semi-fluid state hardens forming the hard yellow yolk we find in a boiled egg. 

However food scientists have found that boiling eggs for different duration of time leads in different extent of denaturation, enabling them to experiment with dishes that have a slightly runny yolk or a slightly soft texture to the boiled egg.