What chemical changes happen to food when it is cooked?

[Trillionaire]

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.