A reversible change is a change that can be undone or reversed, meaning that any changes made can be reversed. These changes are observed in the physical state, size, and shape of the substance. A reversible change does not create a new material. Examples of reversible changes include melting and freezing of water, which can be reversed by cooling or heating respectively. Another example is the change of shape of some solids, such as clay, which can be molded into different forms and then molded back into their original shapes. A blacksmith changing a piece of iron into different tools is also an example of a reversible change. In contrast, irreversible changes cannot be undone, and the material is permanently altered. Examples of irreversible changes include cooking a raw egg, which changes from liquid to solid and cannot be reversed.