Why do eggs become hard when boiled?
Not only eggs, but also meat and fish become tough when cooked or baked.
All animal foods have the property of becoming brittle when heated.
This is because protein, a component of the animal body, has the property of hardening when heated.
All animals, no matter what they are, have protein in their bodies, which is why they become hard when heated. Eggs, in particular, are a food that contains a lot of protein, which is why they become hard.
