Some changes can be turned back so you get exactly what you started with. These are called reversible changes. If you can undo a change and return to the beginning, then it is reversible.

Melting an ice cube is a great example. When ice melts, it becomes water. But if you put that water back in the freezer, it turns into ice again. You end up with the same ice you started with, so melting is reversible.

Many changes of state work this way. A liquid can freeze into a solid and then melt back. Water can boil into steam and then cool back into water. In each case, nothing new is made — the material just changes how it looks.

You can also change the shape of things without making something new. If you squish a ball of clay flat, you can roll it back into a ball. Folding paper or stretching a rubber band are changes you can undo, because the material is still the same.

Even mixing some things is reversible. If you stir salt into warm water, the salt seems to disappear. But if the water evaporates away, the salt is left behind, so you can get your salt back again.

Knowing which changes are reversible helps us in real life. Ice cube trays, freezers, and even melting crayons to make new crayon shapes all use reversible changes. When you can get your material back, you can use it again and again.