* Physical Change: A physical change alters the form or appearance of a substance but doesn't change its chemical composition.

* Chemical Change: A chemical change results in the formation of a new substance with different properties.

When glass breaks, it's simply changing its physical state from one solid form (intact glass) to many smaller solid pieces (broken glass). The chemical makeup of the glass (silicon dioxide, etc.) remains the same.

Here's a helpful way to remember:

* Physical changes can be reversed (you can sometimes glue broken glass back together).

* Chemical changes are usually irreversible (you can't easily turn the ashes from burnt wood back into wood).