Here's why:

* Physical change: Alters the form or appearance of a substance but doesn't change its chemical makeup.

* Chemical change: Results in a new substance with different properties.

When you flatten a penny, you're simply changing its shape. The copper (or copper alloy) that makes up the penny is still present, just in a different form. You haven't created any new substances.

Here are some examples to illustrate the difference:

* Physical change: Melting ice (water changes from solid to liquid, but it's still H2O).

* Chemical change: Burning wood (wood reacts with oxygen to create ash, carbon dioxide, and other substances).

In the case of the penny, the change is physical because it's a change in shape only, and the chemical composition of the penny remains the same.