You have often added substances to water and seen them heated; the question is - do they stay like that forever? Do they go back to what they started like? Can they be made to return to their original state or is the change permanent?
This series of questions will give you a chance to look over some basic changes when materials are heated and cooled.
You notice that an old nail holding your gate post together has a browny-looking substance on its surface. What do you think that is?