A brick is made from clay, often with other materials such as straw mixed in with it. Making a brick is an example of an irreversible reaction. That is, when you heat (fire) clay in a big brick oven, or kiln, then clay slightly melts and fuses together. It also loses all of its water. This change cannot be reversed by adding water, which is why we can use bricks for the outsides of our houses and the houses still stay dry.