Why is pounding a marshmallow physical change?

Pounding a marshmallow is not a physical change, but rather a chemical change.

A chemical change occurs when the chemical composition of a substance is altered. When you pound a marshmallow, you are breaking down the sugar molecules in the marshmallow, causing them to form new bonds with each other and with the air molecules that are present. This results in the marshmallow changing color, texture, and flavor.

A physical change, on the other hand, does not involve a change in the chemical composition of a substance. Physical changes can include changes in size, shape, or state of matter. For example, melting ice is a physical change because the water molecules are simply changing from a solid state to a liquid state.