Why does an egg set when heated?

Egg white and yolk consist largely of proteins. In their raw state, these proteins are usually folded up tightly, like small, twisted balls of yarn.

When heated, the molecules gain more movement energy, and the weak forces holding the ball together break apart – the proteins unfold. The now exposed parts of the proteins can bond with neighboring proteins.

Step by step, this creates a tight network of interconnected proteins that traps water. This exact network is what gives cooked egg white its firm, yet still slightly elastic texture.