* Reversible Change: A change where the original substance can be recovered. Think of melting ice (solid) into water (liquid) and then freezing the water back into ice.

* Irreversible Change: A change where the original substance cannot be easily recovered. The change is permanent or nearly so.

When you break a rubber band, you're physically separating the rubber molecules. While you might be able to glue the pieces back together, the rubber band will never be the same as it was before it was broken. The molecular structure has been permanently altered.