What does air pressure do to marshmallows in a bottle?

When marshmallows are placed in a bottle and the air pressure is reduced (such as when a vacuum is created), the marshmallows expand. This occurs because the lower pressure allows the air trapped within the marshmallows to expand, causing them to puff up. Conversely, when normal air pressure is restored, the marshmallows will return to their original size as the air inside them is compressed. This demonstrates the relationship between air pressure and gas expansion within objects.