There are many ways of changing materials. You can melt some, freeze some, dissolve some and even burn some. Some of these changes can be reversed - you can get back what you started with. But some are irreversible. They are permanent. You can never get back what you started with. You have a completely new material.

Most cooking is like that. Once food is cooked, it is changed irreversibly.

But making a solution can be reversible. You can often get back what you started with.

Some irreversible changes are exciting to watch. Colours change, new materials are produced. There may be bubbling. They may get hot.