Have you ever helped crack an egg, or watched someone else do it? If so, then you’ve seen that what’s inside the egg is gooey and soupy -- an egg white (which is usually clear) and a squishy round yellow yolk in the middle. So why does an egg that has boiled in hot water form into something solid (even if a bit rubbery) that you can hold in your hand!?
The reason why eggs transform shape when they’re heated up (boiled in their shell or even just fried or scrambled in a pan!) has to do with what they’re made up of: Lots of proteins! Proteins are units that make up a large part of most living things, and it’s important for us to eat proteins as part of our diet.
When the proteins of a raw, liquid egg come into contact with heat, it causes them to change form! The reason they change is that the heat makes the individual units of protein link together differently into what are ultimately stronger chains. This leaves them cooked and ready for you to eat!