Final answer:
A marshmallow puffs up when toasted because the heat causes the air bubbles inside it to expand, according to Charles's Law, similar to how hot air balloons rise and bread becomes fluffy.
Step-by-step explanation:
When you toast a marshmallow, it puffs up because of the heat from the fire causing the air bubbles inside the marshmallow to expand. This is due to a scientific concept known as Charles's Law, which states that the volume of a gas increases with its temperature if the pressure is held constant. As the marshmallow heats up, the air inside the bubbles gets hotter, becomes less dense, and occupies more space, which makes the marshmallow expand or puff up. The same principle is observed in hot air balloons, where the hot air inside is less dense compared to the cool air outside, creating a buoyant force that makes them rise. It's similar to how the yeast in freshly-baked bread produces carbon dioxide gas, which expands and creates fluffy bread when heated.