How we did it:
Light a campfire. Then, let the fire die down to a bed of hot coals.
Carefully place a marshmallow into the coals.
Observe what happens. Then, compare it to a fresh marshmallow!
The marshmallow grows when it burns because of chemistry happening deep within it. Marshmallows are made almost entirely out of sugar, and when sugar burns, one of the chemical compounds it makes is water vapor (like the hot steam from a kettle!). The water vapor is what makes the marshmallow grow.
When the burning starts on the surface of the marshmallow, the water vapor can escape out into the air. But when the burning travels into the inside of the marshmallow, the water vapor can’t escape anymore. Instead, it puffs the marshmallow up full of steam like a hot air balloon. Look closely - the hot water vapor makes the surface of the marshmallow bubble, too!