Why does heating food cause its texture, appearance, etc to change? I realise that one of the main reasons we cook food is to kill germs, but what else is going on?
If you cook a potato it gets softer but if you cook an egg it gets harder. Raw meat shrinks when it is cooked, but popcorn expands. Why does heat affect the different molecules in different ways?
I’d like a fairly basic summary to this (one that would satisfy, say, a fairly bright ten year old.)
Thanks.
Heat denatures proteins. Different foods contain different proteins, with different functions (like holding things together stiffly, or contributing to membrane fluidity, for example), so what the heat does to the food will depend partially on what proteins it contains.
Heat also provides the energy for certain chemical reactions to go to completion and make product(s). Different foods are made of different starting materials (reactants), so the heat is driving different reactions and producing different products, resulting in the variable changes in color, texture, etc.
I think that’s pretty basic, but I could be missing something. If so, someone’ll come along & correct me (ah, the beauty of the Dope). 
Lots of different things happen.
Eggs get solid because they have protiens in them that are long molecules floating around independantly, and heat causes them to form occasional bonds with neighbors so they can’t move independantly.
Popcorn expands because the liquid in the cells tries to boil and the pressure bursts the shell, turning what was a solid, wet seed into a porous foam.
There are many different effects and they all happen at rates that depend in diffrent ways on temperature. That’s why you want to cook potato chips in seconds and roasts over hours. There isn’t one main thing that explains all cooking.